fix ignored IOPub messages with no parent

there was a `continue` without a new `recv`,
which would cause an infinite loop.

IPython/parallel/client/client.py

@@ -853,6 +853,7 b' class Client(HasTraits):'
             # ignore IOPub messages with no parent.
             # Caused by print statements or warnings from before the first execution.
             if not parent:
+                idents,msg = self.session.recv(sock, mode=zmq.NOBLOCK)
                 continue
             msg_id = parent['msg_id']
             content = msg['content']