Add PyNaCl to the list of packages that trigger libffi bootstrapping

To reduce confusion for users who have not explicitly listed cffi as a
dependency.

Fixes #142.

Add PyPy 5.0.1

Keep pre-existing environment variables.

`PKG_CONFIG_PATH` variable.

Update pip and setuptools

Update buildpack image to align with marketing

https://pythonhosted.org/setuptools/history.html
https://bitbucket.org/pypa/setuptools/branches/compare/20.2.2%0D19.6

The release on PyPI isn't signed, though the MD5 of this archive matches
that on the PyPI package page:

$ md5sum setuptools-20.2.2.tar.gz
bf37191cb4c1472fb61e6f933d2006b1 *setuptools-20.2.2.tar.gz

https://pip.pypa.io/en/stable/news/
https://github.com/pypa/pip/compare/8.0.2...8.1.0

Archive verified via:
https://pypi.python.org/packages/source/p/pip/pip-8.1.0.tar.gz.asc

And Donald's key from:
https://pypi.python.org/security

$ gpg --verify pip-8.1.0.tar.gz.asc pip-8.1.0.tar.gz
gpg: Signature made Sat, Mar  5, 2016  4:57:30 PM GMT using RSA key ID 3372DCFA
gpg: Good signature from "Donald Stufft (dstufft) <donald@stufft.io>"
gpg:                 aka "Donald Stufft (dstufft) <donald@python.org>"
...
Primary key fingerprint: 7C6B 7C5D 5E2B 6356 A926  F04F 6E3C BCE9 3372 DCFA

https://github.com/heroku/heroku-buildpack-python/compare/v77...c98615cdd00f7525e98524a40a59cf18d7f7a0b6

Correct typos and non-canonical/HTTPS URLs

https://devcenter.heroku.com/articles/buildpacks#officially-supported-buildpacks

To save the 301s.