Add travis config and a badge to the readme

Unfortunately the badge is red :) the bridgewater account will have to log into travis and press the on switch for the scala-notebook repo, but beyond that it should just work.

I'd normally squash the three commits but they're public now and travis has seen them, and I didn't really want to mess with public history (and I couldn't do it privately or travis wouldn't get triggered)

Note that you'll also want to point the readme badge to the right travis account, since it points to mine right now.