Add travis ci support

Runs lein test in stefon-core using travis.

See the successful build here: https://travis-ci.org/RyanMcG/stefon/jobs/14087165.

Of course, just including this file alone would not get travis-ci working in the circleci repo. You would need to do a bit of configuration on the travis side.  At that point we could add a build status image to the README too.