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 `outputPaths.app.html` in your `Broccoli.js` file:

    var EmberApp = require('ember-cli/lib/broccoli/ember-app');

    var app = new EmberApp({
      outputPaths: {
        app: {
          html: 'custom-name.html'
        },
      }
    });

    module.exports = app.toTree();