Set cookbook updated at date

We used to set some cookbook attributes in Cookbook#publish_version but
no longer do. We left save! in to bump Cookbook#updated_at when a new
version was published but it turns out this doesn't happen if no
attributes are changed so this explicitly sets the updated at date to
the current time.
Add migration to resolve incorrect updated at dates

For a period of time we haven't been updating a cookbooks updated at
date when a new version is published. This adds a migration that sets
all cookbook updated at dates to the creation date of the latest
cookbook version.
Merge pull request #645 from opscode/touch-cookbook

Touch cookbook instead of saving