Order cookbook_versions by created_at DESC.

:fork_and_knife: This pull request has some ups and downs. At first take, I thought it made sense to order cookbooks by their version. However, [when looking at the current community site](https://github.com/opscode/opscode-community-site/blob/master/app/models/cookbook.rb#L24), it looks like they are orded by created_at DESC. I can see why this makes sense in some ways, which is fine. The mental issue I am having is [the API docs](http://docs.opscode.com/api_cookbooks_site.html#id1) show the versions ordered by version ASC or created_at ASC (does not specify). The current community site code seems to conflict with the docs at first glance.

In this PR, I have updated the association to order by created_at DESC, just like the current community site. Should I stick with that and assume the docs are wrong or does this warrant a bigger discussion surrounding the ordering of `CookbookVersions`?

Logic path of finding the ordering:

[`latest_version` is an instance method on `Cookbook`](https://github.com/opscode/opscode-community-site/blob/master/app/controllers/api/v1/cookbooks_controller.rb#L94).  [Checking that out, it is just the first `cookbook_version`](https://github.com/opscode/opscode-community-site/blob/master/app/models/cookbook.rb#L54), [which are ordered by `created_at DESC`](https://github.com/opscode/opscode-community-site/blob/master/app/models/cookbook.rb#L24).

Thanks!