Add configuration for travis heroku deploys

This will cause automatic deployment to heroku once all specs on master pass. I followed this [guide](http://docs.travis-ci.com/user/deployment/heroku/) for setting everything up.

Note that migrations and a restart is run on every deploy. If you'd like to avoid migrating on every deploy, then we can have this deploy to staging (with migrations every time) and then promote manually and do migrations only when necessary.