Modify service param in restart zend-server handler

The 'ensure=restarted' parameter for the service module is not a valid parameter.

This fails for me with ansible-playbook v 1.6 (devel b2350d8aaf).

See: http://docs.ansible.com/service_module.html#options

Replacing 'ensure=restarted' with 'state=restarted' resolves the issue.