Fix cookbook platform and dependency access scope

Accessing these relations through a has_one relation results in
displaying all SupportedPlatform and CookbookDependency models
associated with _any_ version of the cookbook.

This feels like it could be a Rails bug, but for the time being, we can
fix it by adding these two simple methods to Cookbook
Merge pull request #301 from opscode/fix-cookbook-dependency-and-platform-query-scope

Fix cookbook platform and dependency access scope