Add Travis CI configuration to validate DCO and gofmt

After each push, Travis CI will trigger, and check two things:

- make sure that each commit in the push has the Docker certificate of origin
- make sure that all .go files changed by this sequence of commits are correctly formatted in the most recent commit

Note: there is one edge case; if you do a git force push, we cannot figure out the actual commits in the force push, and we will just run the checks as if upstream master were the base. Pull requests will always be tested correctly, though.

Docker-DCO-1.0-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3475 from tianon/travis

Add Travis CI configuration to validate DCO and gofmt