Support gnome-shell 3.21.91 (debian testing)

Author: p91paul

metadata.json

@@ -6,5 +6,6 @@
  "shell-version": ["3.14",
                    "3.16",
                    "3.18",
-                   "3.20"]
+                   "3.20",
+                   "3.21.91"]
 }