fix(docs): restore "make zipfile" target for pypi docs

In our [release docs](http://docs.deis.io/en/latest/contributing/releases/#documentation) we mention `make -C docs/ zipfile` which makes an archive of our HTML documentation suitable for uploading to pypi.python.org. That is effectively our backup documentation site: https://pythonhosted.org/deis/

I accidentally refactored out the `make zipfile` target a while ago, so I've done this by hand for a couple releases. This restores the zipfile target and re-alphabetizes the python requirements files for CORS.