mongooseのWarning(DeprecationWarning: `open()` is deprecated in mongoose >= 4.11.0, use `openUri()` instead, or set the `useMongoClient` option if using `connect()` or `createConnection()`)

DeprecationWarning: `open()` is deprecated in mongoose >= 4.11.0, use `openUri()` instead, or set the `useMongoClient` option if using `connect()` or `createConnection()`. See http://mongoosejs.com/docs/connections.html#use-mongo-client

このissueで、「{ useMongoClient: true }」を追加するなどの対策があるようだが。
https://github.com/Automattic/mongoose/issues/5399

結局ダウングレードして対応した。

npm install mongoose@4.10.8 --save