fix(docs): silence header warnings and restore broken links

We had some warnings and a couple broken link errors that had crept into the docs build. This cleans those up, updates some Sphinx-related packages, and removes the now-unused `pexpect` package.

Specifically, this fixes the 2 broken links currently at http://docs.deis.io/en/latest/using_deis/deploy-application/#select-a-build-process