Disable Clang on Linux builds
Disable Clang on Linux builds

llvm.org disabled their APT mirror due to excess load, so Linux hosts on
Travis can't install latest Clang versions and fail. Will re-enable in
the future.

newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,x4121/newsbeuter,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,x4121/newsbeuter,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,x4121/newsbeuter,newsboat/newsboat,x4121/newsbeuter