Travis CI: build using both GCC and Clang
Travis CI: build using both GCC and Clang

On OS X, `gcc` is just an alias for Clang, so exclude it to
avoid wasting resources doing the exact same build job twice.

Akemi/libass,MrSmile/libass,Akemi/libass,MrSmile/libass,astiob/libass,xbmc/libass,astiob/libass,libass/libass,ShiftMediaProject/libass,xbmc/libass,ShiftMediaProject/libass,Akemi/libass,libass/libass