Bump VERSION to 1.0.0

• Diff: https://gitlab.com/gitlab-org/gitlab-dangerfiles/compare/v0.9.0...fb94e9b

• Release notes:

### New features and features updates

- !24 Use the API to retrieve a MR diff and fallback to Git.

• 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.