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?
Such settings are a problem because they are not backwards compatible. If this pull request is merged upstream, tests will fail for everyone. Maybe you could have that, but do not run Sauce Labs tests if settings file do not exist (maybe just warn, so that one can see that this is an option).