Fix deprecation about optional parameter order in LocaleListener

src/Silex/Provider/Locale/LocaleListener.php

@@ -33,7 +33,7 @@ class LocaleListener implements EventSubscriberInterface
     private $requestStack;
     private $requestContext;
 
-    public function __construct(Container $app, $defaultLocale = 'en', RequestStack $requestStack, RequestContext $requestContext = null)
+    public function __construct(Container $app, string $defaultLocale, RequestStack $requestStack, RequestContext $requestContext = null)
     {
         $this->app = $app;
         $this->defaultLocale = $defaultLocale;