Update Configuration for Symfony deprecation (#309)

* config update

* namespaced method

Author: snebes

DependencyInjection/Configuration.php

@@ -20,9 +20,15 @@ class Configuration implements ConfigurationInterface
      */
     public function getConfigTreeBuilder()
     {
-        $treeBuilder = new TreeBuilder();
+        $treeBuilder = new TreeBuilder('lexik_form_filter');
+
+        if (\method_exists($treeBuilder, 'getRootNode')) {
+            $rootNode = $treeBuilder->getRootNode();
+        } else {
+            // BC layer for symfony/config 4.1 and older
+            $rootNode = $treeBuilder->root('lexik_form_filter');
+        }
 
-        $rootNode = $treeBuilder->root('lexik_form_filter');
         $rootNode
             ->children()
                 ->arrayNode('listeners')