Fix for Ender + Backbone

When Backbone is used as an Ender module, `exports` always exists, so
`root.ender` needs to be passed to `factory`. (When not using Ender,
the 4th arg will be undefined, same as before.)

Note that Ender 2.x no longer automatically integrates all modules into
`$`. There are two ways to use Backbone with Ender 2:

1. Assign it to its own var and use it like this:

var Backbone = require('backbone');
Backbone.View.extend({…});

2. Integrate into `$` by adding `--integrate backbone` when building
the Ender library, then use it like this:

$.View.extend({…});