Fix Travis PR testing to not overwrite the "master" that Travis creates with our "upstream master", and to also ignore commits with no content (like a Merge commit - there's no need to sign off on literally nothing)

Docker-DCO-1.0-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3489 from tianon/travis-fixes

Fix Travis PR testing to not overwrite the "master" that Travis creates with our "upstream master"