Upgrade webpack-dev-server

[christianbundy]

This resolves another iteration of #1268 which was resolved in webpack/webpack-dev-server#975. The current error is:

Uncaught Error: Incompatible SockJS! Main site uses: "1.1.4", the iframe: "1.1.2".

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[Timer]

Great, thanks for catching this!
I wish we could be more careful about this but I'm not sure about comments in package.json.

[Timer]

Hi there! This change is out in [email protected]; please give it a go! Thanks.