Fixing length of reference to 50

Fixing length to 50 as described in ekyna/PayumMoneticoBundle#3

Author: Alexandre-T

Diff for: src/Api/Options/RequestResolver.php

@@ -53,7 +53,7 @@ public function __construct()
             ->setAllowedValues('currency', Assert::currency())
             ->setAllowedTypes('reference', 'string')
             ->setAllowedValues('reference', function ($value) {
-                return preg_match('~^[0-9A-Za-z]{1,12}$~', $value);
+                return preg_match('~^[0-9A-Za-z]{1,50}$~', $value);
             });
 
         $this