Fix handler to be more compatible with tornado/pyzmq updates

[kevin-bates]

This addresses build issues related to allowing Notebook 6 support (#321). However, I'm suspicious of jupyter_client 5.3.1 which may be causing this issue near the end of the tests...

Exception in thread Thread-31:
Traceback (most recent call last):
  File "/opt/anaconda3/envs/kernel-gateway-dev/lib/python3.6/threading.py", line 916, in _bootstrap_inner
  File "/opt/anaconda3/envs/kernel-gateway-dev/lib/python3.6/site-packages/jupyter_client/channels.py", line 167, in run
  File "/opt/anaconda3/envs/kernel-gateway-dev/lib/python3.6/site-packages/jupyter_client/channels.py", line 94, in _create_socket
  File "/opt/anaconda3/envs/kernel-gateway-dev/lib/python3.6/site-packages/zmq/sugar/context.py", line 146, in socket
  File "/opt/anaconda3/envs/kernel-gateway-dev/lib/python3.6/site-packages/zmq/sugar/socket.py", line 59, in __init__
  File "zmq/backend/cython/socket.pyx", line 328, in zmq.backend.cython.socket.Socket.__init__
zmq.error.ZMQError: Too many open files

We should revisit this issue once this PR is merged and we can see how PR #321 behaves.