Upgrading es6-promise dependency

lib/axios.js

@@ -1,10 +1,12 @@
-var Promise = require('es6-promise').Promise;
 var defaults = require('./defaults');
 var utils = require('./utils');
 var deprecatedMethod = require('./helpers/deprecatedMethod');
 var dispatchRequest = require('./core/dispatchRequest');
 var InterceptorManager = require('./core/InterceptorManager');
 
+// Polyfill ES6 Promise if needed
+require('es6-promise').polyfill();
+
 var axios = module.exports = function axios(config) {
   config = utils.merge({
     method: 'get',