Fix mongoose 4.11.0 deprecation warning

[nbkhope]

Fixes the following deprecation warning:

(node:3351) 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