Use new WebpackDevServer option (facebook#3401)

Author: gaearon

packages/react-scripts/config/webpackDevServer.config.js

@@ -94,7 +94,7 @@ module.exports = function(proxy, allowedHost) {
     },
     public: allowedHost,
     proxy,
-    setup(app) {
+    before(app) {
       // This lets us open files from the runtime error overlay.
       app.use(errorOverlayMiddleware());
       // This service worker file is effectively a 'no-op' that will reset any

packages/react-scripts/package.json

@@ -54,7 +54,7 @@
     "sw-precache-webpack-plugin": "0.11.4",
     "url-loader": "0.6.2",
     "webpack": "3.8.1",
-    "webpack-dev-server": "2.9.3",
+    "webpack-dev-server": "2.9.4",
     "webpack-manifest-plugin": "1.3.2",
     "whatwg-fetch": "2.0.3"
   },