Bump gatsby from 3.7.2 to 3.9.1

Bumps [gatsby](https://github.com/gatsbyjs/gatsby) from 3.7.2 to 3.9.1.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/compare/[email protected]@3.9.1)

---
updated-dependencies:
- dependency-name: gatsby
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]