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

This is looking pretty good against `master@ 3941908`, 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: still busted from https://github.com/mapbox/mapbox-gl-native/pull/407#issuecomment-51815467 (@incanus)