Switch pip-pop back to using pip 8

Fixes to the library vendoring's 'already exists' checks

Since if someone previously had `GDAL` in their requirements file, they
would already have the gdalserver binary present from `bin/steps/gdal`
but be missing the proj and geos vendor files. By checking for `proj`
instead, we ensure that the vendoring isn't incorrectly skipped in this
case.

For both the cedar and cedar-14 stacks, the libffi directory is named
`libffi-3.1` not `libffi-3.1.1`, so the existing library wouldn't be
detected, causing it to be re-downloaded during every compile.

Since we need to set it regardless of whether the vendored library
existed previously or not.

This means pip-pop will use the buildpack-installed pip 8, which doesn't
fail when encountering requirements files that use the new hash syntax.

Fixes #302.

Is the fix from kennethreitz/pip-pop#12.

Add Python 2.7.12, Python 3.5.2, Pypy 5.3.1, Pip 8.1.2, Setuptools 23.1.0 and remove release candidate

* $ curl
https://pypi.python.org/packages/e7/a8/7556133689add8d1a54c0b14aeff0acb0
3c64707ce100ecd53934da1aa13/pip-8.1.2.tar.gz -o pip-8.1.2.tar.gz
* $ curl
https://pypi.python.org/packages/9f/7c/0a33c528164f1b7ff8cf0684cf88c2e73
3c8ae0119ceca4a3955c7fc059d/setuptools-23.1.0.tar.gz -o
setuptools-23.1.0.tar.gz
* $ ls
* $ rm pip-8.1.1.tar.gz
* $ rm setuptools-20.4.tar.gz

https://www.python.org/downloads/release/python-352/

Runtimes:
* Added pypy-5.3.0
* Moved python-2.7.11rc1 to python-2.7.12rc1

clean up pip-grep in the `not silent` case

remove Pylibmc reference in cryptography comment

Use Travis

Added another package that relies on cffi

The extraneous `format` call is harmless in itself, but it's possible for `requirement.req` to be `None` here, which results in `AttributeError: 'NoneType' object has no attribute 'project_name'`.

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.