Bump es6-promise from 4.2.5 to 4.2.8

Bumps [es6-promise](https://github.com/stefanpenner/es6-promise) from 4.2.5 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](stefanpenner/es6-promise@v4.2.5...v4.2.8)

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