Bump version to 6.4.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.3.0...ae2945d9e567a4b15946ecc1fdcb10afc55782ae

• Release notes:

### Fixes

- !539 Wait up to 5 mins for GitLab to start

### Other changes (CI, backstage)

- !541 Pass CI_JOB_ID along
- !534 Persist the `testcase` URL into the JUnit JSON reports

• 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.