Replace Cursor.nextObject because it was removed from node-mongodb-native library

https://github.com/HolgerFrank/gridfs-stream/commit/9cd82a002bd63c07b5e5b70bf2ef2922527a6a28

Works for me, please pull this.

Temporary fix

mongoose.mongo.Cursor.prototype.nextObject = mongoose.mongo.Cursor.prototype.next;