Add missing method in the Predis\Network\IConnectionCluster interface.

lib/Predis.php

@@ -1169,6 +1169,7 @@ interface IConnectionSingle extends IConnection {
 }
 
 interface IConnectionCluster extends IConnection {
+    public function add(IConnectionSingle $connection);
     public function getConnection(ICommand $command);
     public function getConnectionById($connectionId);
 }