Upgrade `aws-s3` gem to `aws-sdk-s3`

The former has been deprecated for many years now. This should also allow to upload files larger than 5GB

https://github.com/webtranslateit/safe/blob/09908d1ba5f7ded809d6b7be6083beb50fe0b45a/lib/webtranslateit/safe/s3.rb#L7