Add build status for Travis-CI to readme

To have this working (and to get tests running on other pull requests), it requires someone with admin access (@ohnoroboto? @_beechnut) to [sign in](http://docs.travis-ci.com/user/getting-started/#Step-one%3A-Sign-in) and [set up the service hook](http://docs.travis-ci.com/user/getting-started/#Step-two%3A-Activate-GitHub-Service-Hook).  The rest of the steps should be all set.