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.

`PKG_CONFIG_PATH` variable.