Mongoose: aggregate query for showing only most recent document version

I'm trying to create an aggregate query (or any query for that matter) that will only pull the most recent document version. I tried using the suggested aggregate query with $$ROOT but that throws an error and $$ROOT doesn't seem to be available from even the raw mongodb driver

        Post.collection.aggregate([
        { $sort : { uuid : 1, __v: -1}},
        { $group : { _id : "$uuid", doc: {$first: "$$ROOT"}}},
        { $match: {company: company}}
        ], callback);

This throws an error about a field value starting with '$' is not acceptable. Any help would be appreciated.