travis: drop unneeded x11proto-xf86vidmode-dev

The only place where the package is needed is for building the DRI
based libGL library.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Dylan Baker <dylan@pnwbakers.com>
Acked-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 84445a86)
[Juan A. Suarez: resolve trivial conflicts]
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>

Conflicts:
	.travis.yml