allow to initialize list of engines of Differ

example of usage: 

https://github.com/docteurklein/event-store/blob/44a78e9c38ae3524e91dd5623fb743301a07bc03/src/TestWork/PhpSpecExtension/ExceptionStringer.php#L28
This will fail at PHP 5.3 as you used syntax sugar from 5.4+.