Make sure polyfill is loaded first by using import statement

ES6 `import` statements hoist their imports to the top of the file.
Using `require(‘babel/polyfill’)` actually loads the polyfill after the
other imports, causing errors.

js/app.js

@@ -1,6 +1,6 @@
 'use strict';
 
-require('babel/polyfill');
+import polyfill from 'babel/polyfill';
 
 import React from 'react';
 import { Router, Redirect } from 'react-router';