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>
1 file changed