After having upgraded to Ruby 1.9.1 I get the following error:

/volume1/public/s3sync/s3sync.rb:23:in `require': no such file to load -- md5 (LoadError)
        from /volume1/public/s3sync/s3sync.rb:23:in `<module:S3sync>'
        from /volume1/public/s3sync/s3sync.rb:11:in `<main>'

It seems that the new version of Ruby is missing the MD5 library. Can anyone help?