Cannot find Underscore, JQuery, or Backbone CDNs

[rfdickerson]

I could be wrong, but wanted to file an issue before I submit a PR for a fix. It appears that the index.html file in frontend cannot resolve links to the URLs for various Javascript libraries. The following fix seems to work:

<script src="bower_components/todomvc-common/base.js"></script>
        <script src="https://code.jquery.com/jquery-2.2.4.min.js"   integrity="sha256-BbhdlvQf/xTY9gja0Dq3HiwQF8LaCRTXxZKRutelT44="   crossorigin="anonymous"></script>
        <script src="http://underscorejs.org/underscore-min.js"></script>
        <script src="http://backbonejs.org/backbone-min.js"></script>
        <script src="js/models/todo.js"></script>
        <script src="js/collections/todos.js"></script>
        <script src="js/views/todo-view.js"></script>
        <script src="js/views/app-view.js"></script>
        <script src="js/routers/router.js"></script>
        <script src="js/app.js"></script>