use AR 4.x pluck for collection labeling

If you have a collection with hundreds of items, formtastic loads all of them to get the `member_label` and `member_value` of it. That is very slow! Since Rails 4 has a [`pluck`](http://apidock.com/rails/ActiveRecord/Calculations/pluck) method, we can go a more performant way!
With `pluck` only the requested fields are loaded form the DB.

This PR is inspired by @seanlinsley in activeadmin/activeadmin#2166