Call ActiveRecord#distinct instead of custom query

Prevent this error when calling AR#any? (or AR#count) on a collection in
spree/admin/products

   PG::UndefinedColumn: ERROR:  column "count_column" does not exist
   LINE 1: SELECT COUNT(count_column) FROM (SELECT  DISTINCT ON(id,name...
                        ^
   : SELECT COUNT(count_column) FROM (SELECT  DISTINCT ON(id,name) spree_products.* AS count_column FROM "spree_products"  WHERE "spree_products"."deleted_at" IS NULL AND ('t'='t') LIMIT 10 OFFSET 0) subquery_for_count

I'm not really sure why postgres was given special treatment here before. Not
sure what changed inside rails either that made that query invalid. This is
the query generate now:

    SELECT COUNT(DISTINCT count_column) FROM (SELECT  DISTINCT "spree_products"."id" AS count_column FROM "spree_products"  WHERE "spree_products"."deleted_at" IS NULL AND ('t'='t') LIMIT 10 OFFSET 0) subquery_for_count

And this is the query generated in current Rails 4-0-stable:

    SELECT COUNT(count_column) FROM (SELECT 1 AS count_column FROM "spree_products" WHERE "spree_products"."deleted_at" IS NULL AND ('t'='t') LIMIT 10 OFFSET 0) subquery_for_count