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>
diff --git a/.travis.yml b/.travis.yml
index e317a02..061aed1 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,6 +4,7 @@
 dist: trusty
 
 cache:
+  apt: true
   directories:
     - $HOME/.ccache