fixes #414: run Travis on Linux (build & test) & OS X (build OS X & iOS)

This is looking pretty good against [`master`](https://github.com/mapbox/mapbox-gl-native)@[`3941908`](https://github.com/mapbox/mapbox-gl-native/commit/3941908cee9d91ac505908e3443b4490644b2cf7), but Travis is failing on both platforms right now, so that needs fixed first. 

 * ~~Linux: something related to `src/util/raster.cpp` (see https://travis-ci.org/mapbox/mapbox-gl-native/jobs/32369159 /cc @lbud)~~

 * ~~OS X: iOS is still busted from https://github.com/mapbox/mapbox-gl-native/pull/407#issuecomment-51815467 (@incanus)~~

**Edit:** Scratch both of the above as complete/irrelevant. I think the holdup right now is Travis reliability issues; will check on this later.