Prepare release

Author: sebastianbergmann

ChangeLog-5.7.md

@@ -2,7 +2,11 @@
 
 All notable changes of the PHPUnit 5.7 release series are documented in this file using the [Keep a CHANGELOG](http://keepachangelog.com/) principles.
 
-## [5.7.21] - 2017-MM-DD
+## [5.7.21] - 2017-06-21
+
+### Added
+
+* Added `PHPUnit\Framework\AssertionFailedError`, `PHPUnit\Framework\Test`, and `PHPUnit\Framework\TestSuite` to the forward compatibility layer for PHPUnit 6
 
 ### Fixed
 

src/Runner/Version.php

@@ -30,7 +30,7 @@ public static function id()
         }
 
         if (self::$version === null) {
-            $version       = new Version('5.7.20', dirname(dirname(__DIR__)));
+            $version       = new Version('5.7.21', dirname(dirname(__DIR__)));
             self::$version = $version->getVersion();
         }