Fix failing backup filename spec for RCs

This spec was failing on dev: https://gitlab.com/gitlab-org/gitlab-ce/builds/16139416

1) gitlab:app namespace rake task Human Readable Backup Name name has human readable time
     Failure/Error: expect(@backup_tar).to match(/\d+_\d{4}_\d{2}_\d{2}_\d+\.\d+\.\d+(-pre)?_gitlab_backup.tar$/)

       expected "/builds/gitlab-org/gitlab-ce/tmp/tests/backups/1494564739_2017_05_12_9.2.0-rc1_gitlab_backup.tar" to match /\d+_\d{4}_\d{2}_\d{2}_\d+\.\d+\.\d+(-pre)?_gitlab_backup.tar$/
       Diff:
       @@ -1,2 +1,2 @@
       -/\d+_\d{4}_\d{2}_\d{2}_\d+\.\d+\.\d+(-pre)?_gitlab_backup.tar$/
       +"/builds/gitlab-org/gitlab-ce/tmp/tests/backups/1494564739_2017_05_12_9.2.0-rc1_gitlab_backup.tar"
     # ./spec/tasks/gitlab/backup_rake_spec.rb:355:in `block (3 levels) in <top (required)>'

See http://rubular.com/r/9oI7K8b773 for more details.

approved this merge request

@stanhu Thanks! /cc @axil

enabled an automatic merge when the pipeline for 5bed3390 succeeds

canceled the automatic merge

merged

mentioned in commit b1873771

Oops, I forgot the RCs!

Picked into 9-2-stable, will go into 9.2.0-rc2.

mentioned in issue #31934 (closed)

mentioned in commit a2b7252c