Fixes Symfony 4.2 deprecation messages (backwards compatible) (stwe#887)

(cherry picked from commit 5cca7f7)

Author: stephanvierkant

DependencyInjection/Configuration.php

@@ -27,8 +27,13 @@ class Configuration implements ConfigurationInterface
      */
     public function getConfigTreeBuilder()
     {
-        $treeBuilder = new TreeBuilder();
-        $rootNode = $treeBuilder->root('sg_datatables');
+        $treeBuilder = new TreeBuilder('sg_datatables');
+        if (method_exists($treeBuilder, 'getRootNode')) {
+            $rootNode = $treeBuilder->getRootNode();
+        } else {
+            // BC layer for symfony/config 4.1 and older
+            $rootNode = $treeBuilder->root('sg_datatables');
+        }
 
         $this->addDatatableSection($rootNode);