Bump sequelize from 5.7.3 to 5.15.1

Bumps [sequelize](https://github.com/sequelize/sequelize) from 5.7.3 to 5.15.1.
- [Release notes](https://github.com/sequelize/sequelize/releases)
- [Commits](sequelize/sequelize@v5.7.3...v5.15.1)

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

Author: dependabot[bot]