fixes deprecations on the translation provider

src/Silex/Provider/TranslationServiceProvider.php

@@ -13,7 +13,9 @@ namespace Silex\Provider;
 
 use Pimple\Container;
 use Pimple\ServiceProviderInterface;
+use Symfony\Component\HttpKernel\Kernel;
 use Symfony\Component\Translation\Translator;
+use Symfony\Component\Translation\Formatter\MessageFormatter;
 use Symfony\Component\Translation\MessageSelector;
 use Symfony\Component\Translation\Loader\ArrayLoader;
 use Symfony\Component\Translation\Loader\XliffFileLoader;
@@ -77,7 +79,11 @@ class TranslationServiceProvider implements ServiceProviderInterface, EventListe
         }
 
         $app['translator.message_selector'] = function () {
-            return new MessageSelector();
+            if (Kernel::VERSION_ID < 30400) {
+                return new MessageSelector();
+            }
+
+            return new MessageFormatter();
         };
 
         $app['translator.resources'] = function ($app) {