change config knp_menu.enable from scalar to boolean (kevinpapst#143)

Author: tbordinat

DependencyInjection/Configuration.php

@@ -114,7 +114,7 @@ private function getKnpMenuConfig()
         $rootNode
             ->addDefaultsIfNotSet()
             ->children()
-                ->scalarNode('enable')
+                ->booleanNode('enable')
                     ->defaultFalse()
                     ->info('')
                 ->end()