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.

* skip pip install for pipenv

* better

* improvements

* indent

* chmod +x

* export

* skip uninstall too

* pip freeze

* Revert "skip uninstall too"

This reverts commit faac96f62004f78c3d27a92cd865954fc7a3a53d.

* better comments

* redirect stderr to stdout

So that any changes made to `.heroku/` within pre_compile (such as
installing additional libraries required for the later pip install) are
not clobbered by the cache being copied over afterwards.

Fixes #320.

* update python.gunicorn.sh to WEB_CONCURRENCY.sh

* don't remove WEB_CONCURRENCY for prefixed 0s

* split gunicorn functionality into its own file

* cleanup comments

* fix var ordering

* add retry to curl

* use proper url for buildpack stdlib

* basics of metrics

* pin to stdlib v2

* measure python installation size

* lots of additional metrics

* add old output methods

* $

* empty commit

* don't unset DYNO (we're running on Heroku)

Fixes #280

* blacklist STACK env var for subshells