Remove contentBase deprecation warning message

[ened]

Without the change, I had the error message:

22:05:30 hot-assets.1 | started with pid 9115
22:05:31 hot-assets.1 | yarn run v0.24.5
22:05:31 hot-assets.1 | $ (cd client && yarn run hot-assets) 
22:05:31 hot-assets.1 | yarn run v0.24.5
22:05:31 hot-assets.1 | $ NODE_ENV=development babel-node server-rails-hot.js 
22:05:33 hot-assets.1 | Webpack HOT dev build for Rails
22:05:33 hot-assets.1 | Using a URL as contentBase is deprecated and will be removed in the next major version. Please use the proxy option instead.
22:05:33 hot-assets.1 | proxy: {
22:05:33 hot-assets.1 | 	"*": "<your current contentBase configuration>"
22:05:33 hot-assets.1 | }
22:05:33 hot-assets.1 | Using a URL as contentBase is deprecated and will be removed in the next major version. Please use the proxy option instead.

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This change is 

[justin808]

Thanks @ened