Bump VERSION to 5.13.3

This is needed for omnibus-gitlab#5399 (comment 351252615)

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v5.13.2...7f5e5e839d57c0a6b7e2d26695fed1a61cfb26ee

• Release notes:

### Fixes

- !465 Login to docker if we're using Omnibus mirror
- !466 Remove a line which is not needed and accidentally left out

• Checklist before merging:

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

• Checklist after merging:

  • Create a tag for the new release version.