Travis: use Clang 11 for the code coverage build
Travis: use Clang 11 for the code coverage build

This works around https://github.com/rust-lang/rust/issues/76085.

newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat