Merge pull request #108 from nextcloud/update-stable21-target-versions

Update stable21 target versions

Author: nickvergessen

.travis.yml

@@ -9,7 +9,7 @@ services:
 
 env:
   global:
-    - CORE_BRANCH=master
+    - CORE_BRANCH=stable21
     - APP_NAME=files_retention
   matrix:
     - DB=sqlite