Bump VERSION to 0.7.0

• Diff: https://gitlab.com/gitlab-org/gitlab-dangerfiles/compare/v0.6.1...293793f

• Release notes:

### New features and features updates

- !15 Introduce DangerSpecHelper and extract Gitlab::Dangerfiles::Change to a separate file.

• 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.