Fixed deprecated warning

[arumals]

Seems like this fixes the error message

(node:34075) 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

[nbkhope]

Duplicate of #5