Saucelabs support

This pull request is only so that we can do an internal code review. We will not get merge it, but send a pull request upstream.

@arunoda, we are adding Sause Labs support to travis-ci-meteor-packages. Would that be something you would be interested in merging in?
can't you simply use env variables. It's very easy to add env variables via travis.yml. Since this is a public project, I'm quite not sure how to hide keys from public.