travis: disable cache

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

.travis.yml

@@ -2,12 +2,7 @@ os: linux
 dist: bionic
 arch: arm64
 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
+cache: false
 addons:
   firefox: "latest"