Update composer.json (added zend-form repository)

error message when using "zf.php" : 
Fatal error: Class 'Zend\Form\FormElementManager' not found in /usr/local/zend/apache2/htdocs/ZFTool/vendor/zendframework/zend-mvc/Zend/Mvc/Service/AbstractPluginManagerFactory.php on line 33

zend-mvc/Zend/Mvc/Service/FormElementManagerFactory.php
needs zend-form 

added repository in 'composer.json' to resolve dependency :
"zendframework/zend-form": ">=2.0.0",