Update MonologExtension.php

Change 'MongoClient'  Definition to 'MongoDB\Client' - Fixes #420

Author: cimo23

DependencyInjection/MonologExtension.php

@@ -279,7 +279,7 @@ private function buildHandler(ContainerBuilder $container, $name, array $handler
 
                 $server .= $handler['mongo']['host'].':'.$handler['mongo']['port'];
 
-                $client = new Definition('MongoClient', [
+                $client = new Definition('MongoDB\Client', [
                     $server,
                 ]);