fix bug in stale profile clean up for clusters "clusters" tab of my notebook wasn't rendering, so checked the log, found the stack trace: ``` [...] File "/usr/local/lib/python3.4/site-packages/ipython-3.0.0-py3.4.egg/IPython/html/services/clusters/handlers.py", line 21, in get self.finish(json.dumps(self.cluster_manager.list_profiles())) File "/usr/local/lib/python3.4/site-packages/ipython-3.0.0-py3.4.egg/IPython/html/services/clusters/clustermanager.py", line 77, in list_profiles self.update_profiles() File "/usr/local/lib/python3.4/site-packages/ipython-3.0.0-py3.4.egg/IPython/html/services/clusters/clustermanager.py", line 74, in update_profiles self.profiles.pop(stale) TypeError: unhashable type: 'set' ``` looks like a pretty straightforward mistake