Bump es6-promise from 4.2.6 to 4.2.8
Bump es6-promise from 4.2.6 to 4.2.8

Bumps [es6-promise](https://github.com/stefanpenner/es6-promise) from 4.2.6 to 4.2.8.
- [Release notes](https://github.com/stefanpenner/es6-promise/releases)
- [Changelog](https://github.com/stefanpenner/es6-promise/blob/master/CHANGELOG.md)
- [Commits](https://github.com/stefanpenner/es6-promise/compare/v4.2.6...v4.2.8)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
squidfunk/karma-viewport,squidfunk/karma-viewport