.travis.yml: Cache Avocado cache

Avocado tests download artifacts from various sources.
These sources sometime have network issues resulting in build
failures. Cache Avocado cache to reduce build failure.

See https://docs.travis-ci.com/user/caching/#arbitrary-directories



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