Bump mongodb from 2.2.31 to 3.1.13

Bumps [mongodb](https://github.com/mongodb/node-mongodb-native) from 2.2.31 to 3.1.13.
- [Release notes](https://github.com/mongodb/node-mongodb-native/releases)
- [Changelog](https://github.com/mongodb/node-mongodb-native/blob/master/CHANGES_3.0.0.md)
- [Commits](mongodb/node-mongodb-native@v2.2.31...v3.1.13)

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