Remove webpack-dev-server upgrade (#1356)
Remove webpack-dev-server upgrade (#1356)

In the Upgrade section the readme had `yarn upgrade webpack-dev-server --latest`. This will cause issues related to https://github.com/rails/webpacker/issues/1295 until webpacker supports webpack 4.0.
gauravtiwari/webpacker,rossta/webpacker,rossta/webpacker,rossta/webpacker,rails/webpacker,rossta/webpacker,gauravtiwari/webpacker,rails/webpacker,gauravtiwari/webpacker