mongo backend doesn't work with mongdb 3.4

When creating index in mongodb_backend.MongoTable:

pymongo.errors.OperationFailure: The field 'drop_dups' is not valid for an index specification. Specification: { name: "login_1", unique: true, drop_dups: true, key: { login: 1 } }

drop_dups no longer supported as an option to be passed when creating and index in 3.4