.travis.yml: Enable multiple caching features

Using the 'multiple caching features' means explode the YAML array,
thus it eases the git workflow (it is easier to move patches around).

See https://docs.travis-ci.com/user/caching#enabling-multiple-caching-featuresSigned-off-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Message-Id: <20170809202712.6951-4-f4bug@amsat.org>
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>

.travis.yml

@@ -5,7 +5,8 @@ dist: xenial
 language: c
 compiler:
   - gcc
-cache: ccache
+cache:
+  ccache: true
 
 
 addons: