Update webpack and other JS and make hot reloading work

[justin808]

We should update the JS packages to the latest and get hot reloading working again.

[Naaman-Saif]

Hi. I am looking into this.

[justin808]

@Naaman-Saif any news?

[waclock]

Couldn't this be fixed by just adding
'Access-Control-Allow-Origin': '*' wherever the client does the polling and requests for the reloaded asset? I'm not sure where this is in react_on_rails, but it's what they suggest here https://github.com/webpack/webpack-dev-server/blob/master/examples/api/simple/server.js