It appears like the gem is using the aws-sdk-v1 but the dependency is not specific so I got v2 installed which uses a different namespace. Adjusted the gemspec and the require gem name, as well as the README.md to specify the correct gem to install.
I am not sure how this gem ever worked to upload new files to Amazon S3, at least with the aws-sdk-v1 version I have. For me, it kept failing with a "No Such Key" exception raised by the AWS SDK. Added a rescue for that specific error to upload the file if it does not already exist in S3.
I found the same problem. I think the key does not exist error comes up when the file isn't already in S3. AWS can't return the hash of the file in o.etag