[DependencyInjection] Add the Required attribute.

Author: derrabus

composer.json

@@ -28,7 +28,7 @@
     "minimum-stability": "dev",
     "extra": {
         "branch-alias": {
-            "dev-master": "2.1-dev"
+            "dev-master": "2.2-dev"
         },
         "thanks": {
             "name": "symfony/contracts",