Update eslint requirement from 5.12.0 to 5.15.0 (#4394)

Updates the requirements on [eslint](https://github.com/eslint/eslint) to permit the latest version.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/master/CHANGELOG.md)
- [Commits](https://github.com/eslint/eslint/compare/v5.12.0...v5.15.0

)

Signed-off-by: dependabot[bot] <support@dependabot.com>