Change updated date back to Cookbook#updated_at

Turns out that a cookbook is saved when a new version is published so we
should just use Cookbook#updated_at to determine when a cookbook was
most recently updated. Using the CookbookVersion#created_at date seems
good in theory but in practice it makes it a pain to order cookbooks by
most recently updated since then you need to get subqueries involved.
Remove defunct recently_updated scope

The Cookbook.recently_updated scope is no longer in use, let's remove
it.
Merge pull request #509 from opscode/recently-updated

Recently updated