Update mongodb to the latest version 🚀 (parse-community#4855)

* fix(package): update mongodb to version 3.1.0

* chore(package): update lockfile

https://npm.im/greenkeeper-lockfile

* starting mongo 3.1.0, read preferences are passed again

* Adds test confirming parse-community#4831 is properly functional now

Author: greenkeeper[bot]