Bump VERSION to 6.4.0

I decided to do a minor version bump; I'm not sure what we consider a breaking change, because, since this cop is enabled by default, it will now break client repositories unless they add silences?

• Diff: https://gitlab.com/gitlab-org/gitlab-styles/compare/v6.3.0...bc671f90645bb9ac5e33a53c2997201e51950f5b

• Release notes:

### New features and features updates

- !92 Add Style/OpenStructUse cop.

• Checklist before merging:

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

• Checklist after merging:

  • Update the release notes for the newly created tag.