Fix webpack config for Windows (#3926)

config/webpack/loaders/babel.js

@@ -1,7 +1,7 @@
 module.exports = {
   test: /\.js$/,
   // include react-intl because transform-react-remove-prop-types needs to apply to it
-  exclude: /node_modules\/(?!react-intl)/,
+  exclude: /node_modules[\/\\](?!react-intl)/,
   loader: 'babel-loader',
   options: {
     forceEnv: process.env.NODE_ENV || 'development',