Add compatibility for symfony/config 3.4

by fixing
Error: Call to undefined method Symfony\Component\Config\Definition\Builder\TreeBuilder::getRootNode()

src/ServiceContainer/PantherConfiguration.php

@@ -33,7 +33,13 @@ public function addOptionsNode(): ArrayNodeDefinition
     {
         $treeBuilder = new TreeBuilder('options');
 
-        $node = $treeBuilder->getRootNode()
+        if (\method_exists($treeBuilder, 'getRootNode')) {
+            $root = $treeBuilder->getRootNode();
+        } else {
+            $root = $treeBuilder->root('options');
+        }
+
+        $node = $root
             ->info(
                 "These are the options passed as first argument to PantherTestCaseTrait::createPantherClient client constructor."
             )