Add column to cookbook_versions to support README/dependency migration verification

:fork_and_knife: 

This supports the work to be done in this [Trello card](https://trello.com/c/MTTK1bl1). The next version of [chef-legacy](https://github.com/gofullstack/chef-legacy) will include a task that combs through each migrated `CookbookVersion` to make sure its README and its dependencies were migrated correctly. It'll use this new column to track the state of this process.