bug #594 Fix deprecation for symfony/config 4.2 (mhujer)

This PR was merged into the 5.2.x-dev branch.

Discussion
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Fix deprecation for symfony/config 4.2

Fixed the same way as in doctrine/DoctrineBundle#853

Closes #593

Commits
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7db9568 Fix deprecation for symfony/config 4.2

DependencyInjection/Configuration.php

@@ -29,8 +29,14 @@ class Configuration implements ConfigurationInterface
      */
     public function getConfigTreeBuilder()
     {
-        $treeBuilder = new TreeBuilder();
-        $rootNode = $treeBuilder->root('sensio_framework_extra', 'array');
+        $treeBuilder = new TreeBuilder('sensio_framework_extra');
+
+        if (method_exists($treeBuilder, 'getRootNode')) {
+            $rootNode = $treeBuilder->getRootNode();
+        } else {
+            // BC layer for symfony/config 4.1 and older
+            $rootNode = $treeBuilder->root('sensio_framework_extra');
+        }
 
         $rootNode
             ->children()