[travis] do not build clang on osx
[travis] do not build clang on osx

Summary:
This restores the functionality from before D5208622: we didn't use to build
clang on osx because Travis does not allow enough time for the build process to
finish.

Reviewed By: mbouaziz

Differential Revision: D5292233

fbshipit-source-id: 0b6573d

jvillard/infer,facebook/infer,jvillard/infer,facebook/infer,blzq/infer,ropas/infer,facebook/infer,blzq/infer,jvillard/infer,blzq/infer,ropas/infer,blzq/infer,facebook/infer,facebook/infer,ropas/infer,facebook/infer,jvillard/infer,ropas/infer,facebook/infer,blzq/infer,jvillard/infer,jvillard/infer,ropas/infer,jvillard/infer,blzq/infer,ropas/infer