Updated identity check example to use correct method call when sending a ZMQ::Message

The correct method in the latest version of the `ffi-rzmq` gem used to send a message via a socket is `#sendmsg`.

See http://rdoc.info/gems/ffi-rzmq/ZMQ/CommonSocketBehavior#sendmsg-instance_method for more info.
Merge pull request #351 from alakra/master

Updated identity check example to use correct method when sending a message