use JSON.stringify for definePlugin

Author: nitin42

webpack/webpack.config.js

@@ -26,7 +26,7 @@ const plugins = () => [
     debug: false
   }),
   new webpack.DefinePlugin({
-    'process.env.NODE_ENV': 'production'
+    'process.env.NODE_ENV': JSON.stringify('production')
   }),
   new webpack.optimize.ModuleConcatenationPlugin(),
   new UglifyJSPlugin()