Fix Travis for PRs against non-master (like release, for example)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3652 from tianon/fix-travis-on-non-master-prs

Fix Travis for PRs against non-master (like release, for example)
Update Travis to also test compiling the docs

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3656 from tianon/travis-test-docs

Update Travis to also test compiling the docs
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)