travis: upgrade to use Clang 3.7
travis: upgrade to use Clang 3.7

Clang 3.7 has been released and we finally have a new APT repository for
it.  Yeah!  :D

Ableton/atria,AbletonAG/atria,AbletonAG/atria,Ableton/atria,dodheim/atria,AbletonAG/atria,Ableton/atria,dodheim/atria,dodheim/atria