Reverted model group id changes

Author: GrahamCampbell

lib/Gitlab/Model/Project.php

@@ -1144,7 +1144,7 @@ public function removeSnippet($id)
     }
 
     /**
-     * @param int|string $group_id
+     * @param int $group_id
      *
      * @return Group
      */

lib/Gitlab/Model/User.php

@@ -215,7 +215,7 @@ public function removeKey($id)
     }
 
     /**
-     * @param int|string $group_id
+     * @param int $group_id
      * @param int        $access_level
      *
      * @return User
@@ -228,7 +228,7 @@ public function addToGroup($group_id, $access_level)
     }
 
     /**
-     * @param int|string $group_id
+     * @param int $group_id
      *
      * @return bool
      */