prepared the 2.2.1 release

doc/changelog.rst

 Changelog
 =========
 
-2.2.1 (2017-XX-XX)
+2.2.1 (2017-12-14)
 ------------------
 
 * added support for Swiftmailer SSL stream_context_options option

src/Silex/Application.php

@@ -41,7 +41,7 @@ use Silex\Provider\HttpKernelServiceProvider;
  */
 class Application extends Container implements HttpKernelInterface, TerminableInterface
 {
-    const VERSION = '2.2.1-DEV';
+    const VERSION = '2.2.1';
 
     const EARLY_EVENT = 512;
     const LATE_EVENT = -512;