travis: radv needs LLVM 4.0

This is a backport for 18.0 from 6ce400782c ("travis: radeonsi and radv
need LLVM 4.0") that fixes Travis build with meson + vulkan.

CC: 18.0 <mesa-stable@lists.freedesktop.org>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Andres Gomez <agomez@igalia.com>