Add Travis configuration file

This is pretty basic, but it's a good start IMO.  As-is, it'll test whether we still successfully compile for the three platform+arch combinations we officially support, and we can grow it from there.

The `sudo: false` bit might not work immediately after enabling Travis for the boot2docker-cli repo (which can be enabled by a repo administrator at https://travis-ci.org/profile/boot2docker), but it might/should work soon (I've requested that the boot2docker org be added to the beta testers list for that).