travis: enable apt cache

Provides a small, but consistent improvement.
Example numbers of the jobs added later in the series.

"make loaders/classic DRI" - 1s
"scons SWR" - 6s

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>

.travis.yml

@@ -4,6 +4,7 @@ sudo: false
 dist: trusty
 
 cache:
+  apt: true
   directories:
     - $HOME/.ccache