Revert "travis: disable cache"

This reverts commit eb87b07d519592b8863a3aeb39eb0a758083f5cc.

.travis.yml

@@ -2,7 +2,12 @@ os: linux
 dist: bionic
 arch: arm64
 language: python
-cache: false
+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"