Add Firefox to the Travis test matrix

[jphalip]

Currently we only test with PhantomJS. For more completeness, it'd be useful to add Firefox following the steps here: https://docs.travis-ci.com/user/gui-and-headless-browsers/#Using-xvfb-to-Run-Tests-That-Require-a-GUI

[cgoldberg]

+1 for adding Chrome also

[jmbowman]

Since both Chrome and Firefox have been added to Travis, I presume this can be closed now.

[jphalip]

That's right, thanks.