Bump gray-matter from 4.0.1 to 4.0.2

Bumps [gray-matter](https://github.com/jonschlinkert/gray-matter) from 4.0.1 to 4.0.2.
- [Release notes](https://github.com/jonschlinkert/gray-matter/releases)
- [Changelog](https://github.com/jonschlinkert/gray-matter/blob/master/CHANGELOG.md)
- [Commits](jonschlinkert/gray-matter@4.0.1...4.0.2)

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