Thanks for the PR, but you actually don't need to use ERB for your S3 keys. Deployment to S3 automatically reads your environment variables as follows.
ENV['AWS_ACCESS_KEY_ID']
ENV['AWS_SECRET_ACCESS_KEY']
ENV['AWS_DEFAULT_REGION']
ENV['AWS_DISTRIBUTION_ID']
See the code here.
Originally posted by @imathis in #62 (comment)