support mongo 3.6++

[cescoferraro]

I have issues trying to authenticate with the database using version > 3.4.9

Db.prototype.authenticate method will no longer be available in the next major release 3.x as MongoDB 3.6 will only allow auth against users in the admin db and will no longer allow multiple credentials on a socket. Please authenticate using MongoClient.connect with auth credentials.

[nrobert13]

I'm using mongo:4.0 and getting the same warning, but it works. It would be nice to implement the new method to avoid the flood of warning. Something like: Automattic/mongoose#5304