Auto-deploy master merges to heroku

[brycefisher]

Currently, deploying the latest version of the repo to heroku must be done from a developer's machine manually. In an ideal world, the heroku toolbelt would be installed in the Travis build (via the .travis.yml file) and credentials would be added to the Travis pipeline via the Settings UI for the project. We could check the git branch to see if we're building master, and then run the deploy command.