Use Travis

Added another package that relies on cffi

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