Bump backstopjs from 6.0.1 to 6.0.4 (#804)

Bumps [backstopjs](https://github.com/garris/backstopjs) from 6.0.1 to 6.0.4.
- [Release notes](https://github.com/garris/backstopjs/releases)
- [Changelog](https://github.com/garris/BackstopJS/blob/master/changelog.md)
- [Commits](https://github.com/garris/backstopjs/commits)

---
updated-dependencies:
- dependency-name: backstopjs
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]