.travis.yml: Increase cache timeout from 3min to 20min

We are going to cache few gigabytes, increase the cache timeout
to avoid build failures when uploading our cache.

See https://docs.travis-ci.com/user/caching/#setting-the-timeout

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

.travis.yml

@@ -6,6 +6,7 @@ language: c
 compiler:
   - gcc
 cache:
+  timeout: 1200
   ccache: true