Update webpack.config.js

OccurenceOrderPlugin has been renamed to OccurrenceOrderPlugin.
webpack/webpack#1964

Author: dyaroman

webpack.config.js

@@ -17,7 +17,7 @@ module.exports = {
     publicPath: '/static/'
   },
   plugins: [
-    new webpack.optimize.OccurenceOrderPlugin(),
+    new webpack.optimize.OccurrenceOrderPlugin(),
     new webpack.HotModuleReplacementPlugin(),
     new NpmInstallPlugin()
   ],