Update webpack version note (facebook#2798)

* Remove note about webpackHotDevClient being webpack 1.0 only

It must work in webpack 2 since create-react-app is still using it and is using webpack 2 now.

It would be great if you could add some kind of note about how it differs from the default webpack hot reloaders.

* Update README.md

* Update README.md

packages/react-dev-utils/README.md

@@ -295,7 +295,7 @@ Returns an object with local and remote URLs for the development server. Pass th
 
 This is an alternative client for [WebpackDevServer](https://github.com/webpack/webpack-dev-server) that shows a syntax error overlay.
 
-It currently supports only Webpack 1.x.
+It currently supports only Webpack 3.x.
 
 ```js
 // Webpack development config