Remove the explicit -> { order("categories.position ASC") } condition from :feeds association

Because the SQL generated will delegate ordering to categories association
which generate SQL `GROUP BY categories.position ASC`. Having this condition
explicitly declared does not do anything, the SQL still generate `GROUP BY feeds.position`.
Merge pull request #228 from joneslee85/refactor-paper-feeds-association

Remove the explicit -> { order("categories.position ASC") }