Fix compile error for libzmq 4.2.1 and 4.2.2 (zeromq#161)

* Fix compile error for libzmq 4.2.1 and 4.2.2

When defined ZMQ_BUILD_DRAFT_API=1, you receive an error "'on_event_handshake_succeeded' was not declared in this scope"... which is correct for versions 4.2.1 and 4.2.2.
Function was renamed from 'on_event_handshake_succeed', fixed this by updating the function name.

zmq.hpp

@@ -872,7 +872,7 @@ namespace zmq
                 on_event_handshake_failed(*event, address.c_str());
                 break;
             case ZMQ_EVENT_HANDSHAKE_SUCCEED:
-                on_event_handshake_succeeded(*event, address.c_str());
+                on_event_handshake_succeed(*event, address.c_str());
                 break;
 #endif
 #endif