Test with libraries with removed features.

[Robpol86]

Make sure docs that import from the project don't break when features are removed. Example: v1.0.0 of a Sphinx extension has .. customDirective: and the docs use it as an example. Then v2.0.0 it was renamed to .. custom_directive and the docs in that version were updated. Make sure v1.0.0 still builds.

Might need to use inheriting virtualenvs: http://stackoverflow.com/questions/12079607/make-virtualenv-inherit-specific-packages-from-your-global-site-packages