travis-ci: disable clang on Linux
travis-ci: disable clang on Linux

Travis-CI's Linux environment with clang seems to be broken currently,
and it's not adding any real value for us to build with it, so drop that
build.

OS X covers compilation with clang (not with Linux headers, obviously,
but good enough for most warnings).

narfblock/narfblock,narfblock/narfblock,narfblock/narfblock