Setup Travis CI

Travis is a service for [continuous integration](https://en.wikipedia.org/wiki/Continuous_integration) (entirely free for public projects on GitHub). tl;dr: It automatically builds project and runs tests for you after pushing to the repository.

**How to enable:**
1. Go to https://travis-ci.org/
1. Click “Sign in with GitHub” at the top-right
1. Allow Travis CI to access your GitHub account
1. Go to https://travis-ci.org/profile
1. Click “Sync now”, if necessary, to list all your GitHub projects
1. Click the “Repositories” tab
1. Click the on/off switch next to the repository you want to test
1. Next commits pushed to the repository will be automatically build on Travis.