Use select instead of pluck on Project.group_ids

Former-commit-id: 37cb2e78

app/models/project.rb

@@ -390,7 +390,7 @@ class Project < ActiveRecord::Base
     end
     def group_ids
-      joins(:namespace).where(namespaces: { type: 'Group' }).pluck(:namespace_id)
+      joins(:namespace).where(namespaces: { type: 'Group' }).select(:namespace_id)
     end
   end