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>