Create public/uploads directory if missing when backup is executed

By default there is no gitlab/public/uploads folder when no attachments are uploaded in any project. This breaks the backup, which supposes that the public/uploads directory exists:

    sudo -u git -H bundle exec rake gitlab:backup:create RAILS_ENV=production
    ...
    Dumping uploads ... 
    rake aborted!
    No such file or directory - /home/git/gitlab/public/uploads

This fix creates the public/uploads folder in  Backup::Uploads.dump if non-existant.