travis: update libva required version
Commit fa328456 added VP9 config support, but this needs a newer libva version, 1.7.0 or above. Fixes: fa328456 ("st/va: add VP9 config to enable profile2") CC: 18.1 <mesa-stable@lists.freedesktop.org> Reviewed-by:Emil Velikov <emil.velikov@collabora.com>