update async.js answers to Promises

[gitmihalis]

Regarding issue #104, I used Promises to answer the asynchronous tests. Vanilla JS to the rescue!

[rmurphey]

This is great. To land this, I think we'd want to:

• remove the jquery dependency from the project
• consider shipping a Promise polyfill -- maybe not needed given support

I'd also love to see the XHR converted to use fetch, but after thinking about it I decided that's outside the scope of this PR.

[gitmihalis]

Did remove the jQuery dependency from the js-assesments project to coincide with the new async answers.

[bennowak]

Why hasn't this been merged yet?