Fixing broken comparator test in IE8

Found this while I was playing around with Grunt & jshint for #2344. There was one failing test in IE8 (oh, the humanity) due to a [comparator function missing a return statement](https://github.com/documentcloud/backbone/blob/master/test/collection.js#L1040). Since that's not what's being tested, I added the `return` and IE8 passes once again.

Along the way I also found an errant comma and removed it. It wasn't breaking anything in IE8, but it's what got me to check the test suite there in the first place. Valid syntax++