Enable Travis CI

This adds a `.travis.yml` file for use with [Travis CI](https://travis-ci.org/)

To get Travis CI working:

- Go to https://travis-ci.org/ and sign in with your GitHub credentials
- Then, go to [your profile](https://travis-ci.org/profile) (on the Travis CI website) and enable CI for this repository
- This will give you **free** continuous integration, automatic pull request builds, and a build status indicator

Ideally, you want to follow the instructions *before* merging this PR, so that the merge will get picked up and built by Travis CI.