Use Gitlab::VERSION to version backups

Previous to this commit, backups were tied to the git revision (SHA1) of
the app at the time the backup:create command was invoked. If the SHA1
at the time of restore was different, the script would refuse to restore
the backup.  This commit loosens this tie so that the backup script only
complains if the value of the Gitlab::VERSION constant is different
between the time of backup and the time of restore.