Updated streaming client to RxPHP 2.0

Author: WyriHaximus

src/StreamingClient.php

@@ -69,7 +69,7 @@ protected function stream(ObservableInterface $observable, callable $listener)
                     )
                 )
             );
-        })->subscribeCallback(
+        })->subscribe(
             $listener,
             function ($error) {
                 throw $error;