Add empty mock for http2 (facebook#5686)

Author: kjin

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

@@ -641,6 +641,7 @@ module.exports = function(webpackEnv) {
       dgram: 'empty',
       dns: 'mock',
       fs: 'empty',
+      http2: 'empty',
       net: 'empty',
       tls: 'empty',
       child_process: 'empty',