Bump sinon from 4.2.0 to 4.2.1

Bumps [sinon](https://github.com/sinonjs/sinon) from 4.2.0 to 4.2.1.
- [Release notes](https://github.com/sinonjs/sinon/releases?after=v4.2.0)
- [Changelog](https://github.com/sinonjs/sinon/blob/master/docs/changelog.md)
- [Commits](https://github.com/sinonjs/sinon/compare/v4.2.0...v4.2.1)