flush output before showing tracebacks

closes  #3700

IPython/kernel/zmq/zmqshell.py

@@ -501,6 +501,9 b' class ZMQInteractiveShell(InteractiveShell):'
         self.payload_manager.write_payload(payload)
 
     def _showtraceback(self, etype, evalue, stb):
+        # try to preserve ordering of tracebacks and print statements
+        sys.stdout.flush()
+        sys.stderr.flush()
 
         exc_content = {
             u'traceback' : stb,