Prepare the 2.15.6 release

Author: fabpot

src/Environment.php

@@ -38,12 +38,12 @@
  */
 class Environment
 {
-    public const VERSION = '2.15.6-DEV';
+    public const VERSION = '2.15.6';
     public const VERSION_ID = 21506;
     public const MAJOR_VERSION = 2;
     public const MINOR_VERSION = 15;
     public const RELEASE_VERSION = 6;
-    public const EXTRA_VERSION = 'DEV';
+    public const EXTRA_VERSION = '';
 
     private $charset;
     private $loader;