multi-os

Thanks to @springmeyer's prior art in https://github.com/mapbox/mapbox-gl-native/pull/307.

Is there anything useful that could be pulled in from https://github.com/mapbox/mapbox-gl-native/blob/master/scripts/setup_travis.sh or cleaned up in https://github.com/mapbox/node-fontnik/blob/multi-os/.travis.yml?

Assuming this will let us kill off https://github.com/mapbox/node-fontnik/tree/travis-osx branch.

http://docs.travis-ci.com/user/multi-os/