Update CircleCI Config
Update CircleCI Config

- Update config to version 2.1 to improve error messages.
- Use recommended `cimg` docker images for faster spin up time.
- Remove workflow declaration, this is not required when there is a
  single job in the config file.
- Remove boilerplate comments.
- Add link to config reference page https://circleci.com/docs/2.0/configuration-reference/

marcomorain/rate-limited-logger,Swrve/rate-limited-logger,marcomorain/rate-limited-logger,Swrve/rate-limited-logger