add request_store gem

Hi there,

first of all, I want to thank you for this very useful auditing tool :zap:.

Going through the code I didn't immediately see any code to guarantee that Thread.current will always be used by just one request at a time. That's why I figured it might be useful to add the request_store gem to your project (https://github.com/steveklabnik/request_store).

Let me know if you want anything changed :)

Tom