bug #97 Fix forward compatibility with Symfony 5 (sstok)

This PR was merged into the 2.0-dev branch.

Discussion
----------

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT

Commits
-------

d4014f6 Fix forward compatibility with Symfony 5

src/DependencyInjection/Configuration.php

@@ -19,7 +19,13 @@ class Configuration implements ConfigurationInterface
     public function getConfigTreeBuilder()
     {
         $treeBuilder = new TreeBuilder('rollerworks_password_strength');
-        $rootNode = $treeBuilder->root('rollerworks_password_strength'); // BC for Symfony < 4.2
+
+        if (method_exists($treeBuilder, 'getRootNode')) {
+            $rootNode = $treeBuilder->getRootNode();
+        } else {
+            // BC for Symfony < 4.2
+            $rootNode = $treeBuilder->root('rollerworks_password_strength');
+        }
 
         $rootNode
             ->addDefaultsIfNotSet()