Switch to mongo client to support mongo 3.x

[lukywong]

When i try to use a mongo version 3.6 with auth to run the fixtures, and error comes out, so i just switch to MongoClient to connect followed mongodb instructions

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.
{ MongoError: Authentication failed.
    at Function.MongoError.create (~/xxx/node_modules/mongodb-core/lib/error.js:31:11)
    at ~/xxx/node_modules/mongodb-core/lib/connection/pool.js:497:72
    at authenticateStragglers (~/xxx/node_modules/mongodb-core/lib/connection/pool.js:443:16)
    at Connection.messageHandler (~/xxx/node_modules/mongodb-core/lib/connection/pool.js:477:5)
    at Socket.<anonymous> (~/xxx/node_modules/mongodb-core/lib/connection/connection.js:333:22)
    at emitOne (events.js:116:13)
    at Socket.emit (events.js:211:7)
    at addChunk (_stream_readable.js:263:12)
    at readableAddChunk (_stream_readable.js:250:11)
    at Socket.Readable.push (_stream_readable.js:208:10)
  name: 'MongoError',
  message: 'Authentication failed.',
  ok: 0,
  errmsg: 'Authentication failed.',
  code: 18,
  codeName: 'AuthenticationFailed' }

[Narven]

Is this going to be review accepted? any plans on this?