Remove hardcoded `index.html` value

This allows the generated html file to be renamed however is needed.

This is exceptionally helpful for configuring how the root HTML file
should be cached, since the `index.html` is cached harshly by most web
servers

To use it, configure `applicationHTMLRoot` in your `Broccoli.js` file:

    var app = new EmberApp({
      applicationHTMLRoot: 'custom_name.html'
    })