Patch ActiveRecord 3 `exists?` with scopes

AcitveRecord 3 will delegate `exists?` to the `Relation` rather than
`ActiveRecord::Base` if it is called on a scope, meaning that
`SomeModel.some_scope.exists?` will run a query on the master

/cc @zendesk/quokka @osheroff @tsturge @grosser @shajith @steved555