:scissors: scoped for Rails 4 compat

This is required for upgrading discourse to work on 4.1, because `scoped` is deprecated in Rails 4. I traced the commits, there are d87389b3 and f99acebd to eliminate these calls, so I am guessing this is just introduced by accident in 77b59b54. Anyhow, I ran the tests on Rails 4 and everything is green :)

cc @salbertson