chore(deps): bump mongodb from 2.8.2 to 3.1.0

Bumps [mongodb](https://github.com/mongodb/mongo-rust-driver) from 2.8.2 to 3.1.0.
- [Release notes](https://github.com/mongodb/mongo-rust-driver/releases)
- [Commits](mongodb/mongo-rust-driver@v2.8.2...v3.1.0)

---
updated-dependencies:
- dependency-name: mongodb
  dependency-version: 3.1.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]