fixed : rootNode for configuration of the TreeBuilder

I applied the basic and well-known trick for this part :
check if the new method getRootNode() exist and got for the old root() method for compatibility.

DependencyInjection/Configuration.php

@@ -10,15 +10,21 @@
  */
 class Configuration implements ConfigurationInterface
 {
+    const ROOT_NAME = 'fos_message';
+
     /**
      * Generates the configuration tree.
      *
      * @return TreeBuilder
      */
     public function getConfigTreeBuilder()
     {
-        $treeBuilder = new TreeBuilder('fos_message');
-        $rootNode = $treeBuilder->root('fos_message');
+        $treeBuilder = new TreeBuilder(self::ROOT_NAME);
+        if (\method_exists($treeBuilder, 'getRootNode')) {
+            $rootNode = $treeBuilder->getRootNode();
+        } else {
+            $rootNode = $treeBuilder->root(self::ROOT_NAME);
+        }
 
         $rootNode
             ->children()