Add GitLab version to backup file name

What does this MR do?

Adds GitLab version to backup file name.

• Previously: 1493106747_2017_04_25_gitlab_backup.tar
• Now: 1493107263_2017_04_25_9.1.0_gitlab_backup.tar

Are there points in the code the reviewer needs to double check?

• Check if backup/restore works as intended with the new behaviour
• Check if old backup files work as intended

Testing locally:

1. On master branch create backup:

   bundle exec rake gitlab:backup:create

2. On gl-version-backup-file branch create backup:

   bundle exec rake gitlab:backup:create

3. On gl-version-backup-file branch check that both restore filenames work:

   Old backup filename:

   bundle exec rake gitlab:backup:restore BACKUP=1493106747_2017_04_25

   Make sure it worked.

   ---

   New backup filename:

   bundle exec rake gitlab:backup:restore 1493107263_2017_04_25_9.1.0-pre

   Make sure it worked

Why was this MR needed?

Make it easier to find out what version of GitLab is included in a backup file. Otherwise you need to hack your way into it https://stackoverflow.com/questions/26035262/how-to-find-version-of-gitlab-backup.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?