.travis.yml: Cache Python PIP packages

We always install the same packages ever and ever, cache them.

See https://docs.travis-ci.com/user/caching/#pip-cache



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

.travis.yml

@@ -8,6 +8,7 @@ compiler:
 cache:
   timeout: 1200
   ccache: true
+  pip: true
 
 
 addons: