Because no version is specified of the aws-sdk-ruby gem here: https://github.com/JonathanSerafini/chef-cfn/blob/master/recipes/ohai.rb#L3, the newer version 3 is going to be installed.
Therefore this fails:
https://github.com/JonathanSerafini/chef-cfn/blob/master/files/default/ohai_plugins/cfn.rb#L38
with [2017-12-01T10:36:11+00:00] ERROR: cfn: failed to fetch instance: uninitialized constant Aws::EC2
As far as I can tell there's 2 different ways of fixing this. I don't know which solution you prefer otherwise I would've created a PR ;-)

• Version-lock the aws-ruby-sdk gem to < 3
  Or according to https://github.com/aws/aws-sdk-ruby#upgrading-guide:
• Require the whole SDK: require 'aws-sdk'
• Or just the classes that are needed: require 'aws-sdk-ec2'