Make assertCapturedSuccess display stdout if stderr was empty (#390)

In cases where there is no stderr there was previously no output,
which made it hard to find the cause. The assertion messages have
also been adjusted to remove the repetition (shunit2 already outputs
expected vs actual itself).

Before:
```
testPipenv
ASSERT:Expected captured exit code to be 0; was <2> expected:<0> but was:<2>
testPipenvVersion
```

After:
```
testPipenv
ASSERT:Captured exit code - expected:<0> but was:<2>
 !     Warning: Your application is missing a Procfile. This file tells Heroku how to run your application.
 !     Learn more: https://devcenter.heroku.com/articles/procfile
-----> Installing python-2.7.13
-----> Installing pip
-----> Generating 'requirements.txt' with pipenv

testPipenvVersion
```

Fixes #389.