don't skip Pipenv installation if there are git deps (heroku#656)

Pipenv uninstall, and other improvements (heroku#650)

Update python.gunicorn.sh (heroku#628)

Merge branch 'master' of github.com:heroku/heroku-buildpack-python

# Please enter a commit message to explain why this merge is necessary,
# especially if it merges an updated upstream into a topic branch.
#
# Lines starting with '#' will be ignored, and an empty message aborts
# the commit.

Signed-off-by: Kenneth Reitz <[email protected]>

Merge branch 'master' of github.com:heroku/heroku-buildpack-python

changelog

Signed-off-by: Kenneth Reitz <[email protected]>

Improvements (heroku#471)

* only do this on heroku-16

* history

Signed-off-by: Kenneth Reitz <[email protected]>

* code comment

Signed-off-by: Kenneth Reitz <[email protected]>

update the changelog (heroku#470)

Undo sub_env for pip install, it was a terrible idea (heroku#467)

* fixed the bug for pypy-5.8.0

* update changelog

Signed-off-by: Kenneth Reitz <[email protected]>

* blacklist for old apps

Signed-off-by: Kenneth Reitz <[email protected]>

* only blacklist pythonhome/path

Signed-off-by: Kenneth Reitz <[email protected]>

* updated changelog

Signed-off-by: Kenneth Reitz <[email protected]>

* be more agressive with blacklisting

Signed-off-by: Kenneth Reitz <[email protected]>

* sub_env was a terrible idea

Signed-off-by: Kenneth Reitz <[email protected]>

* changelog update

Signed-off-by: Kenneth Reitz <[email protected]>

Blacklist for old apps (heroku#466)

* fixed the bug for pypy-5.8.0

* update changelog

Signed-off-by: Kenneth Reitz <[email protected]>

* blacklist for old apps

Signed-off-by: Kenneth Reitz <[email protected]>

* only blacklist pythonhome/path

Signed-off-by: Kenneth Reitz <[email protected]>

* updated changelog

Signed-off-by: Kenneth Reitz <[email protected]>