Fixing issue with standalone build

closes #47

lib/axios.js

@@ -5,7 +5,14 @@ var dispatchRequest = require('./core/dispatchRequest');
 var InterceptorManager = require('./core/InterceptorManager');
 
 // Polyfill ES6 Promise if needed
-require('es6-promise').polyfill();
+(function () {
+  // webpack is being used to set es6-promise to the native Promise
+  // for the standalone build. It's necessary to make sure polyfill exists.
+  var P = require('es6-promise');
+  if (P && typeof P.polyfill === 'function') {
+    P.polyfill();
+  }
+})();
 
 var axios = module.exports = function axios(config) {
   config = utils.merge({