Intermittent websocket proxying problem on startup

[andrewosh]

As @olgabot discovered, there seems to be a websocket proxying issue that causes ipywidgets to fail to load when an app is first launched, but additional rerunning of the problem cell will work correctly. This is a very intermittent issue and is difficult to replicate.

First, we should update the proxy pod so that it's using the latest changes from version 1.2.0 of the configurable-http-proxy.

Further stress testing of the proxy server should be done by launching multiple instances of a single application involving interactive widgets (if that's the problem) in rapid succession -- that should hopefully lead to WS errors in the developer console, or additional errors in the proxy-server pod logs.