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.

Right now, it just makes sure the DCO exists and is in the correct format.  We can add verification of the github handle as a future addition.  Also, there's still some debate about whether we want to use `@handle` at the end, because that effectively means you get an email every _single_ time _any_ of your commits is pushed to github in any form, even if it's you doing the pushing.