Bump mysql2 from 0.5.6 to 0.5.7 (#758)

Bumps [mysql2](https://github.com/brianmario/mysql2) from 0.5.6 to 0.5.7.
- [Release notes](https://github.com/brianmario/mysql2/releases)
- [Commits](brianmario/mysql2@0.5.6...0.5.7)

---
updated-dependencies:
- dependency-name: mysql2
  dependency-version: 0.5.7
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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