Fixed Circle CI cache for Ruby version.
Fixed Circle CI cache for Ruby version.

Was missing the checksum which would cause Ruby to be downloaded and
rebuilt each time.

The Docker image has been updated to use an unspecified version to
reduce version maintenance.

The `CI_RUBY_VERSION` environment variable was introduced to be
dynamically set based on value in the `.ruby-version` for improved
maintainability.

bkuhlmann/style_guides