Fix docs manpage generation

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Add Dockerfile man page generation from our Dockerfile reference page

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Update Travis to also compile the man page(s) now that they compile properly again

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Fix Travis build errors by bumping our python module versions to be newer

See also https://bitbucket.org/birkenfeld/sphinx/issue/1281/sphinx-113-not-build-proper-manpages

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3650 from tianon/fix-doc-man-generation

Fix docs man page generation and add Dockerfile man page