.travis.yml: enable ccache for the builds

Travis support ccache on a cache-per-branch basis. Given not much of the
build changes between pushes as well as the duplication in each build it
seems worthwhile enabling this.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Tested-by: David Gibson <david@gibson.dropbear.id.au>

.travis.yml

@@ -5,6 +5,7 @@ python:
 compiler:
   - gcc
   - clang
+cache: ccache
 addons:
   apt:
     packages: