Fix Travis build: clang differs on ubuntu/osx.
Fix Travis build: clang differs on ubuntu/osx.

The problem here is that installing clang-3.8 on ubuntu doesn't make
/usr/bin/clang point to that version... it keeps pointing to the old 3.5
version. We try to manually override CC and CXX to fix this.

f0rki/cb-multios,f0rki/cb-multios,f0rki/cb-multios,f0rki/cb-multios,f0rki/cb-multios