Remove duplicate Cookbook Dependency records

The migration looks a little different than a usual migration. This
approach ought to allow us to remove the data migration implementation
class -- Supermarket::Migration::RemoveDuplicateCookbookDependencies --
from the repo without affecting `rake db:migrate` which as of now Travis
still runs every time it builds the project.
Cookbook versions cannot have duplicate dependencies
Merge pull request #461 from opscode/deduplicate-cookbook-dependencies

Deduplicate cookbook dependencies