travis: update libva required version

Commit fa328456e8f29 added VP9 config support, but this needs a newer
libva version, 1.7.0 or above.

Fixes: fa328456e8f ("st/va: add VP9 config to enable profile2")
CC: 18.1 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
1 file changed