Merge pull request liip#478 from karser/fix-root-node-deprecation

Fix root node deprecation in symfony/config > 4.1

Author: alexislefebvre

src/DependencyInjection/Configuration.php

@@ -26,8 +26,15 @@ class Configuration implements ConfigurationInterface
      */
     public function getConfigTreeBuilder(): TreeBuilder
     {
-        $treeBuilder = new TreeBuilder();
-        $rootNode = $treeBuilder->root('liip_functional_test', 'array');
+        if (method_exists(TreeBuilder::class, 'getRootNode')) {
+            $treeBuilder = new TreeBuilder('liip_functional_test');
+            $rootNode = $treeBuilder->getRootNode();
+        } else {
+            // BC layer for symfony/config 4.1 and older
+            $treeBuilder = new TreeBuilder();
+            $rootNode = $treeBuilder->root('liip_functional_test', 'array');
+        }
+
         $rootNode
             ->children()
                 ->arrayNode('cache_db')