fix: use polyfilled Promise (#496)

Author: leeyeh

src/cloudfunction.js

@@ -1,5 +1,6 @@
 const _ = require('underscore');
 const { _request, request } = require('./request');
+const Promise = require('./promise');
 
 module.exports = function(AV) {
   /**

src/utils/index.js

@@ -1,6 +1,7 @@
 const _ = require('underscore');
 const request = require('superagent');
 const debug = require('debug')('leancloud:request');
+const Promise = require('../promise');
 
 let requestsCount = 0;