updated VERSION for 2.0.21

Author: fabpot

src/Symfony/Component/HttpKernel/Kernel.php

@@ -61,7 +61,7 @@ abstract class Kernel implements KernelInterface
  const MAJOR_VERSION = '2';
  const MINOR_VERSION = '0';
  const RELEASE_VERSION = '21';
- const EXTRA_VERSION = 'DEV';
+ const EXTRA_VERSION = '';
 
  /**
  * Constructor.