Fix root node deprecation in symfony/config > 4.1 (#54)

Author: karser

DependencyInjection/Configuration.php

@@ -13,8 +13,14 @@ class Configuration implements ConfigurationInterface
      */
     public function getConfigTreeBuilder()
     {
-        $treeBuilder = new TreeBuilder();
-        $rootNode = $treeBuilder->root('bm_backup_manager');
+        if (method_exists(TreeBuilder::class, 'getRootNode')) {
+            $treeBuilder = new TreeBuilder('bm_backup_manager');
+            $rootNode = $treeBuilder->getRootNode();
+        } else {
+            // BC layer for symfony/config 4.1 and older
+            $treeBuilder = new TreeBuilder();
+            $rootNode = $treeBuilder->root('bm_backup_manager');
+        }
 
         $rootNode
             ->children()