Update README.md

Fixed bug: 

Zend\Stdlib\Exception\InvalidCallbackException: Invalid callback provided; not callable in /Users/andrew/Sites/hobart/website/vendor/zendframework/zendframework/library/Zend/Stdlib/CallbackHandler.php on line 63

Callbacks should be public!
Merge pull request #79 from dronchik/patch-2

Update README.md