Remove use of event priorities

Event priorities are no longer in Laravel - see dbbfc62bef

Updated the AbstractPolicy terminology to reflect the new behaviour,
which is that there is no guarantee that the catch-all methods will run
after all specific methods have run globally. This behaviour is only
guaranteed within the policy.

src/Group/GroupPolicy.php

@@ -26,7 +26,7 @@ class GroupPolicy extends AbstractPolicy
      * @param string $ability
      * @return bool|null
      */
-    public function after(User $actor, $ability)
+    public function can(User $actor, $ability)
     {
         if ($actor->hasPermission('group.'.$ability)) {
             return true;