The commit (8887402ca81f3d4c63c10126577861711475e80c) changed the way options are treated, but that resulted in passing :raw parameter to the server which results in a failure with the error:

Database command 'mapreduce' failed:

{"ok"=>0.0, "errmsg"=>"unknown m/r field for sharding: raw"}

(Mongo::OperationFailure)
/usr/local/lib/ruby/gems/1.9.1/gems/mongo-1.3.1/lib/mongo/db.rb:506:in `command'
/usr/local/lib/ruby/gems/1.9.1/gems/mongo-1.3.1/lib/mongo/collection.rb:576:in `map_reduce'

I've sent a github pull request with the fix.