.travis.yml: Enable ccache on OSX

By default, ccache is not installed on macOS environments.

See https://docs.travis-ci.com/user/caching/#ccache-on-macos

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
1 file changed