With inspiration from @KevinBrolly, this patch uses the stack image
SQLite3 package but also still providing the dev headers and binary that
users may still be using today. The benefit is that we won't need to
rebuild all the python binaries for this to take affect. We can just
stop shipping SQLite3 from future binaries. In addition, we don't need
to worry about what version and when to update SQLite3 and maintaining
the packages ourselves.

This also includes updates to Python 2.7.15 and Python 3.6.6 so they can
rebuilt with the stack image dev headers instead of building our own
vendored SQLite3.

v137 changelog

Prevent Python 3.7 from being unsupported

Add heroku-18 to our Travis CI config

- 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

Python 3.7.0 is supported but not preferred given how new it is. As a
result, we don't want it to be the default, but we also don't want users
to be confused when upgrading to it.

Closes gh-728

Steal the java buildpack's release script

This will make releasing new versions easier for us.

Closes gh-723

Build Python 3.6.6 and 3.7.0

Also update our documentation and CHANGELOG for this version of the
buildpack.

Add runtime build recipes for our newly released versions

Closes gh-720
Closes gh-721

Updated README for python 3.6.5 and CHANGELOG for v135

Bump pipenv version

Fix new shellcheck warnings breaking periodic CI

New warnings were found in our periodic (CRON) CI jobs.

Fixes https://travis-ci.org/heroku/heroku-buildpack-python/jobs/383404864

Let's test re-enabling shellcheck in CI

Caught these with the re-added shellcheck linting.

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

Use the appropriate encoding to read a Pipfile.lock so we can
properly load the data.

Closes #678

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

* 3.6.5

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

* fix tests

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

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

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

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

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

* Add Python 2.7.15 to the list of runtimes

Closes #691

* Update the default Python 2 to 2.7.15 everywhere

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