Maintenance: Bump mysql2 from 0.5.3 to 0.5.4
Bumps [mysql2](https://github.com/brianmario/mysql2) from 0.5.3 to 0.5.4.
- [Release notes](https://github.com/brianmario/mysql2/releases)
- [Commits](https://github.com/brianmario/mysql2/compare/0.5.3...0.5.4)