minor #324 Fix deprecation for symfony/config 4.2+ (Jibbarth)

This PR was merged into the 1.0-dev branch.

Discussion
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Fix deprecation for symfony/config 4.2+

Fix deprecation for symfony/config 4.2

Fixed the same way as in doctrine/DoctrineBundle#853

Commits
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560d03c Fix deprecation for symfony/config 4.2+

src/DependencyInjection/Configuration.php

@@ -21,8 +21,13 @@ class Configuration implements ConfigurationInterface
      */
     public function getConfigTreeBuilder()
     {
-        $treeBuilder = new TreeBuilder();
-        $rootNode = $treeBuilder->root('maker');
+        $treeBuilder = new TreeBuilder('maker');
+        if (\method_exists($treeBuilder, 'getRootNode')) {
+            $rootNode = $treeBuilder->getRootNode();
+        } else {
+            // BC layer for symfony/config 4.1 and older
+            $rootNode = $treeBuilder->root('maker');
+        }
 
         $rootNode
             ->children()