title: recreate_versions! doesn't work with fog/s3 when version already exists
body: <p>I've got a mounted uploader FileUploader which has a number of versions. A number of TIFF images have been uploaded, and the present versions haven't been correctly converted to JPEG/PNG, though their filenames are correct.</p>
  
  <p>Having updated the FileUploader to correctly convert to JPEG/PNG, recreating versions works flawlessly on the local filesystem. When using the remote images on S3, even using the guide (<a href="https://github.com/carrierwaveuploader/carrierwave/wiki/How-to%3A-Recreate-and-reprocess-your-files-stored-on-fog" rel="nofollow">https://github.com/carrierwaveuploader/carrierwave/wiki/How-to%3A-Recreate-and-reprocess-your-files-stored-on-fog</a>), any images that already exist don't get updated, though their timestamps do. If the versions stored on S3 are deleted, then the versions are created correctly.</p>