Bug introduced with git commit: http://github.com/mongodb/mongo-ruby-driver/commit/c25f1f5997c49ee660c5606a35d64509ab633196

lib/mongo/collection.rb line 482:
opts.assert_valid_keys(:query, :sort, :limit, :finalize, :keeptemp, :verbose, :raw)

This needs to allow :out as a valid key. As such, it is not possible to specify the target map reduce collection name with the current ruby driver.