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

Add argon2 to crypotgraphy step

- Add argon2-cffi
- Add django[argon2] option from Django docs:
  https://docs.djangoproject.com/en/1.10/topics/auth/passwords/#argon2-usage