For better code specifications (#144)

There is no need to use `Promise.prototype['finally']`

src/finally.js

@@ -3,7 +3,7 @@
 var Promise = require('./core.js');
 
 module.exports = Promise;
-Promise.prototype['finally'] = function (f) {
+Promise.prototype.finally = function (f) {
   return this.then(function (value) {
     return Promise.resolve(f()).then(function () {
       return value;