Fix process proxy leak

While looking into other leaks (in this case memory), it was discovered
that the process proxy instance was being leaked across kernel cycles.
This change addresses that particular leak.

Note: Other PRs have also been submitted to address leaks in
`jupyter_client` and `notebook`.  These are:

[PR 360 - Fix memory leak of kernel Popen object](jupyter/jupyter_client#360)
[PR 361 - Fix memory leak of IOLoopKernelManager object](jupyter/jupyter_client#361)
[PR 3424 - Fix memory leak of iopub object in activity monitoring](jupyter/notebook#3424)

enterprise_gateway/services/kernels/remotemanager.py

@@ -169,6 +169,7 @@ def cleanup(self, connection_file=True):
         # which then prevents process proxy cleanup.
         if self.process_proxy:
             self.process_proxy.cleanup()
+            self.process_proxy = None
         return super(RemoteKernelManager, self).cleanup(connection_file)
 
     def get_connection_info(self, session=False):