Patch Webpack config to ignore xor|props plugins

This works around a react-live issue (reported as FormidableLabs/react-live/issues/5) that was blocking the 'yarn buid' task for Gatsby, which in turn blocks the Netlify setup.

www/gatsby-node.js

@@ -12,8 +12,12 @@
 'use strict';
 
 const {resolve} = require('path');
+const webpack = require('webpack');
 
 exports.modifyWebpackConfig = ({ config, stage }) => {
+  // See https://github.com/FormidableLabs/react-live/issues/5
+  config.plugin('ignore', () => new webpack.IgnorePlugin(/^(xor|props)$/));
+
   config.merge({
     resolve: {
       root: resolve(__dirname, './src'),