travis: automatically manage ccache caching

According to the manual

"If you are using ccache, use:

  language: c # or other C/C++ variants

  cache: ccache

to cache $HOME/.ccache and automatically add /usr/lib/ccache to your
$PATH."

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>
(cherry picked from commit 6431b98c54)

.travis.yml

@@ -5,8 +5,7 @@ dist: trusty
 
 cache:
   apt: true
-  directories:
-    - $HOME/.ccache
+  ccache: true
 
 addons:
   apt:
@@ -47,7 +46,6 @@ env:
     - BUILD=scons
 
 install:
-  - export PATH="/usr/lib/ccache:$PATH"
   - pip install --user mako
 
   # Since libdrm gets updated in configure.ac regularly, try to pick up the