[fix] travis: Permission denied: '/home/travis/.cache/pip/...'

Change owner https://docs.travis-ci.com/user/caching/#caches-and-read-permissions
and remove old log in the *before_caching* phase:

  https://docs.travis-ci.com/user/caching/#before_cache-phase

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

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

.travis.yml

@@ -5,7 +5,9 @@ language: python
 cache:
   - directories:
     - $HOME/.cache/pip
-
+before_cache:
+    - sudo chown -R travis:travis $HOME/.cache/pip
+    - rm -f $HOME/.cache/pip/log/debug.log
 addons:
   firefox: "latest"