Ensure cache is clear before upgrading

If we changed some namespace / classes, we need to refresh the cache before anything else.

docs/en/user/upgrade-2.0.x-2.0.y.rst

@@ -8,6 +8,7 @@ The last release is published on https://www.wallabag.org/pages/download-wallaba
 
 ::
 
+    rm -rf var/cache/*
     git fetch origin
     git fetch --tags
     git checkout 2.0.8