- Add stage to Travis CI config and update tests.sh script to recognize
  it

- Update tests to assert there is no Python 2 on Heroku-18

- Update nltk fixture to use Python 3.6 so we can test it on all stacks

Closes gh-730

shellcheck is a truly valuable utility. Unfortunately installing the
packages from debian-sid introduces a conflict. Let's try another
suggestion from: https://github.com/koalaman/shellcheck/wiki/TravisCI

Refs
https://github.com/heroku/heroku-buildpack-python/commit/a0275888a26b8f0132c7bb9fe3bf194b5c4fab1f

* freeze pip version, upon installation

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* use PIP_UPDATE environment variable

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* disable shell checking (for now)

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* --disable-pip-version-check

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

Reduces the impact on Travis runtime caused by #438, by:
* Making the shellcheck job run on the container infra (`sudo: false`)
  which has faster boot times.
* Running the heroku-16 and cedar-14 stack tests in parallel, since
  previously they were separate stages (which are run in series by
  design).

Reduces Travis end-to-end time from ~10 minutes to ~6 minutes.

* fixed the bug for pypy-5.8.0

* enable shellcheck in makefile

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* make compile script shellcheck compatible

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* let's see what this says

* fix pip installation

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* shellcheck detect script

* shellcheck release

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* shellcheck test-compile

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* shellcheck warnings/utils

* updated makefile

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* shellcheck collectstatic

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* shellcheck cryptography

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* shellcheck eggpath fixes

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* shellcheck gdal

* shellcheck goes-libs

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* shellcheck mercurial

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* nltk shellcheck

* shellcheck pip-install

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* shellcheck pip-uninstall

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* shellcheck pipenv

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* shellcheck pylibmc

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* shellcheck python

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* update makefile

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* final update to makefile

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* fix tests

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* actually fix the tests

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* update pipenv-python-version

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* requested fixes

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* update spellcheck sources

* attempted fix for shell globbing

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* further attempted fix

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* switch nltk styling

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* no more need for temp app for installing pip

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* attempt at staged build for shellcheck

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* attempted fix of travis file

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* reorder travis file

* add shellcheck to install

* make things clearer in travis file

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* better travis file

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* attempt at better travis file

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* better travis file

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

commit 60a2a9d0720693b9438b0f1c75f570dc96bf3a4b
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Fri Aug 1 14:58:29 2014 -0400

    silent mode

commit 20368e741993a2c96739aa1de457618a5272bfe7
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Fri Aug 1 14:56:00 2014 -0400

    pylibmc

commit 8a512995d806ad58e4a8cab9cd712e762277dcf3
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Fri Aug 1 14:55:25 2014 -0400

    fix for pip-grep

commit 0b83a0ac11ee67f08dc843b01ecfe72c9b9c2938
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Fri Aug 1 14:50:35 2014 -0400

    cat that shit

commit 27095d320f4e6c0a6bccec945efbdb6f09598cfc
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Fri Aug 1 14:49:11 2014 -0400

    don't silence the output

commit db15ac279169467e1e78cfcc55cb740660d4b4c9
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Fri Aug 1 14:46:55 2014 -0400

    hello

commit 6b851815b07331bf185f218c40d62123df048b7b
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Fri Aug 1 14:41:37 2014 -0400

    cleanup

commit b2ccf76ccb244976e5ee71164cfe591925cb13a4
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Fri Aug 1 14:41:14 2014 -0400

    remove last debugging statement

commit fdcb7a210327f01fdc49dba00c3928cbb4778c72
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Fri Aug 1 14:39:10 2014 -0400

    remove debugging statements

commit dd58a898e4e5244f3b78ce0402537a1b730a3cc3
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Fri Aug 1 14:36:02 2014 -0400

    bp watch

commit c722f6e964b2a25f91d153c65f0c473ff22ff4c8
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Fri Aug 1 14:34:34 2014 -0400

    install finished

commit 81644ba1956798c06f55a8ce4c86239ca4ee8036
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Fri Aug 1 14:33:04 2014 -0400

    install started

commit 62d0e3b81a5ac25e8ceab735e25cf2bb8d161f0b
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Thu Jul 31 14:54:59 2014 -0400

    debug

commit 1775a1b206780fd99d013ca7c0cb8d8de17a8461
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Thu Jul 31 13:01:15 2014 -0400

    test0

commit 3d20cf89ae0f9436a54c456779a544a3b9d226ac
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Thu Jul 31 12:57:07 2014 -0400

    test2/2

commit 751d7496893bc84430a93cd479511786bce9abb9
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Thu Jul 31 12:50:11 2014 -0400

    silent mode

commit f927b7b3dee18c6ee58b36a3f96ad66e53b8d46d
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Thu Jul 31 12:44:58 2014 -0400

    pylibmc with pip-grep

commit 00dfc36701bf72191f0ad7b7d5533ba7a9ac3b10
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Thu Jul 31 12:41:11 2014 -0400

    no debug echo

commit 38d78d800583fc89100cadc551ce715c5800f0b8
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Thu Jul 31 12:40:06 2014 -0400

    source, not subenv

commit f455de19a8c1150aa2a4df164b90c2dc9a195942
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Thu Jul 31 12:36:35 2014 -0400

    don't print "installing dependencies with pip" twice

commit e72ed7ddbafa0722d2ac6f0cdaf4ac4d0ca0f634
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Thu Jul 31 12:35:47 2014 -0400

    refactor

commit 64819d9fd7ac042ef56499750b47214eb863f8d2
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Thu Jul 31 12:24:26 2014 -0400

    pip-grep

commit 5e075dd7dbeda9084ea7ce0151dc17924251c258
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Thu Jul 31 12:24:13 2014 -0400

    no more travis

commit bc9baba04436dc7b8aefeaee24e820a9720ee9e8
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Tue Jul 29 13:32:40 2014 -0400

    if not requirement

commit 0918d781e50e6190d1045c19c5c5a37e00d669dc
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Tue Jul 29 13:28:28 2014 -0400

    only use requirments that are truthy

commit 4f0f750407eae6bc7faea50c7b607a10a11ad1c5
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Tue Jul 29 13:25:15 2014 -0400

    new version of pip-diff

commit d13b397a21733d0a0a5e31c4995dc6a3b65cd977
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Tue Jul 29 13:12:23 2014 -0400

    ignore errors for pip-diff

commit bb09fb0d5051f69a35e086030b11fd39d1539f84
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Tue Jul 29 12:55:55 2014 -0400

    uninstall the packages

commit 29b2e1f2dbe7761e3f7f55827d0adcd0025aeb4b
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Tue Jul 29 12:47:21 2014 -0400

    maybe this one should be relative

commit 2448558624fdce634350e904eaad5fd3aa725c22
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Tue Jul 29 12:44:04 2014 -0400

    docopt

commit 47010d297330869516f6add46ace97a41cdccb8d
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Tue Jul 29 12:40:36 2014 -0400

    first pass at pip-pop integration

commit 59ac5912e1322be771d1969687891ebeb9f4b5f0
Author: Kenneth Reitz <me@kennethreitz.com>
Date:   Tue Jul 22 14:42:08 2014 -0400

    copy requirements