fix custom webpack devServer config

Author: linhaobin

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

@@ -110,7 +110,7 @@ const requireFile = require('./utils/requireFile');
 requireFile(paths.customWebpackDevServerConfig, rewiredConfig => {
   module.exports = function(proxy, allowedHost) {
     const config = defaultConfig(proxy, allowedHost);
-    rewiredConfig(config, proxy, allowedHost);
+    return rewiredConfig(config, proxy, allowedHost);
   };
 });
 // @remove-on-eject-end