Add a travis-ci config file

This'll run `lein test` on travis and report failures on pull requests
and such.

Travis docs here: http://about.travis-ci.org/docs/

Clojure-specific travis docs here:
http://about.travis-ci.org/docs/user/languages/clojure/

Some things that might need to be added:
- add stuff so travis tests against different JDKs?
- add stuff so travis tests against different versions of clojure?