Supported platform cleanup

:fork_and_knife: 

This introduces a new join model to connect `SupportedPlatform` and `CookbookVersion` and defines a rake task to go through and eliminate all the existing duplicates.

In a forthcoming PR, I'll be adding a unique constraint to the db and AR model to ensure `SupportedPlatform` is unique based on name and version, and removing the `cookbook_version_id` column from `SupportedPlatform`.