Bump VERSION to 0.8.1

• Diff: https://gitlab.com/gitlab-org/gitlab-dangerfiles/compare/v0.8.0...879e946

• Release notes:

### Fixes

- !20 Ensure BaseLinter doesn't crash on commit subject with only a 'draft' prefix.

### Other changes (tooling, technical debt)

- !20 Add missing specs.

• Checklist before merging:

  • Diff link is up-to-date.
  • Based on the diff, lib/gitlab/dangerfiles/version.rb is updated, according to SemVer.
  • Release notes are accurate.

• Checklist after merging:

  • Update the release notes for the newly created tag.