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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.

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.

Add PyPy 5.0.1

Keep pre-existing environment variables.