Avoid deprecated notice when using symfony/config > 4.2 (#273)

Author: Olivier Dolbeau

DependencyInjection/Configuration.php

@@ -33,8 +33,13 @@ public function __construct(ContainerBuilder $container)
      */
     public function getConfigTreeBuilder()
     {
-        $treeBuilder = new TreeBuilder();
-        $root = $treeBuilder->root('translation');
+        $treeBuilder = new TreeBuilder('translation');
+        // Keep compatibility with symfony/config < 4.2
+        if (!method_exists($treeBuilder, 'getRootNode')) {
+            $root = $treeBuilder->root('translation');
+        } else {
+            $root = $treeBuilder->getRootNode();
+        }
 
         $this->configsNode($root);
         $this->addAutoTranslateNode($root);