Refactor Travis config

I'm not entirely sure how Travis is going to react to dynamic environment variables, but I'm going to go ahead and give it a shot.

I'll also look into getting SlimerJS working with this new format. I'm not sure if we should wait until SlimerJS has headless support or if we could use [this](http://about.travis-ci.org/docs/user/gui-and-headless-browsers/).

Any input would be greatly appreciated.