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 (see https://github.com/dotcloud/docker/pull/2091);
- 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 on the last 10 commits in the pushed branch. Pull
requests will always be tested correctly, though.

Docker-DCO-1.0-Signed-off-by: Jérôme Petazzoni <jp@enix.org>