refactor: Remove deprecation support.

We are taking advantage of bumping to a major version to remove deprecation support.
From version 5.5 of sensio/framework-extra-bundle the Symfony Config component has a minimal version of 4.3.
Which means `getRootNode()` function is available.

Related to #21.

composer.json

@@ -13,7 +13,7 @@
     "require": {
         "php": "^7.1.0",
         "psr/http-factory": "^1.0",
-        "sensio/framework-extra-bundle": "^5.4",
+        "sensio/framework-extra-bundle": "^5.5",
         "symfony/psr-http-message-bridge": "^1.2",
         "woohoolabs/yin": "^4.0"
     },

src/DependencyInjection/Configuration.php

@@ -15,13 +15,7 @@ class Configuration implements ConfigurationInterface
     public function getConfigTreeBuilder()
     {
         $treeBuilder = new TreeBuilder('qp_woohoolabs_yin');
-
-        if (\method_exists($treeBuilder, 'getRootNode')) {
-            $rootNode = $treeBuilder->getRootNode();
-        } else {
-            // BC layer for symfony/config 4.1 and older
-            $rootNode = $treeBuilder->root('qp_woohoolabs_yin');
-        }
+        $rootNode = $treeBuilder->getRootNode();
 
         $rootNode
             ->children()