Merge pull request #181 from koic/ci_against_ruby_2_7

CI against Ruby 2.7

Author: koic

.circleci/config.yml

@@ -35,6 +35,9 @@ workflows:
       - rake_default:
           name: Ruby 2.6
           image: circleci/ruby:2.6
+      - rake_default:
+          name: Ruby 2.7
+          image: circleci/ruby:2.7
       - rake_default:
           name: Ruby HEAD
           image: rubocophq/circleci-ruby-snapshot:latest # Nightly snapshot build