Prepare 4.2.1 release [ci skip]

Author: lcharette

app/defines.php

@@ -10,7 +10,7 @@
 namespace UserFrosting;
 
 // Some standard defines
-define('UserFrosting\VERSION', '4.2.0');
+define('UserFrosting\VERSION', '4.2.1');
 define('UserFrosting\DS', '/');
 define('UserFrosting\PHP_MIN_VERSION', '5.6');
 define('UserFrosting\PHP_RECOMMENDED_VERSION', '7.1');