minor #1968 don't use the deprecated interface with Symfony 4.2 (xabbuh)

This PR was merged into the 2.4-dev branch.

Discussion
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don't use the deprecated interface with Symfony 4.2

#SymfonyConHackday2018

Commits
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9d9671c don't use the deprecated interface with Symfony 4.2

Author: xabbuh

DependencyInjection/Configuration.php

@@ -53,8 +53,13 @@ public function __construct($debug)
      */
     public function getConfigTreeBuilder()
     {
-        $treeBuilder = new TreeBuilder();
-        $rootNode = $treeBuilder->root('fos_rest', 'array');
+        $treeBuilder = new TreeBuilder('fos_rest');
+
+        if (method_exists($treeBuilder, 'getRootNode')) {
+            $rootNode = $treeBuilder->getRootNode();
+        } else {
+            $rootNode = $treeBuilder->root('fos_rest');
+        }
 
         $rootNode
             ->children()