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?
Now that I am thinking, `package.json` is quite common name and you could override something here. It is possible to have some other name and then use it for `npm install`?