feat(NotificationBundle) Adding guzzle compatibility.

 * Adding compatibility with guzzle >=6.0 for symfony 4.

Author: Yansell Rivas Diaz

composer.json

@@ -21,7 +21,7 @@
     "require": {
         "php": ">=5.6",
         "jms/serializer-bundle": ">=2.0",
-        "guzzlehttp/guzzle": "~6.0",
+        "guzzlehttp/guzzle": ">=6.0",
         "symfony/framework-bundle": "~2.8|~3.0|~4.0"
     }
 }