fix(deps): update mongodb requirement from 2.0.0 to 3.1.0

Updates the requirements on [mongodb](https://github.com/mongodb/mongo-rust-driver) to permit the latest version.
- [Release notes](https://github.com/mongodb/mongo-rust-driver/releases)
- [Commits](mongodb/mongo-rust-driver@v2.0.0...v3.1.0)

---
updated-dependencies:
- dependency-name: mongodb
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Cargo.toml

@@ -19,7 +19,7 @@ console = "0.15.0"
 mysql = "25.0.0"
 regex = "1.4.3"
 semver = "1.0.4"
-mongodb = { version = "2.0.0", features = ["sync"], default-features = false }
+mongodb = { version = "3.1.0", features = ["sync"], default-features = false }
 bson = "2.0.0"
 humanesort = "0.1.0-alpha"