Update .travis.yml

[jywarren]

Make sure these boxes are checked before your pull request is ready to be reviewed and merged. Thanks!

• tests pass -- see README.md for how to run them
• code is in uniquely-named feature branch, and has been rebased on top of latest master (especially if you've been asked to make additional changes)
• pull request are descriptively named
• if possible, multiple commits squashed if they're smaller changes
• reviewed/confirmed/tested by another contributor or maintainer

Please be sure you've reviewed our contribution guidelines at https://publiclab.org/wiki/contributing-to-public-lab-software

Please alert developers on [email protected] when your request is ready or if you need assistance.

Thanks!