since old stacks aren't supported

This reverts commit 8e42040d.

* drop duplicate mkdir for .heroku

* simplify linking and moving of python and source

* steps/python needs its own symlink setup

* fix symlink destinations for vendor and venv

* always symlink python

Upgrade to pip-9.0.1 and setuptools-28.8.0

Add PyPy2 v5.6.0

This would REQUIRE that the latest versions are placed into:
https://github.com/heroku/heroku-buildpack-python/tree/master/vendor

https://pypi.python.org/pypi/pip
https://pypi.python.org/pypi/setuptools