Merge pull request #30 from koic/ci_against_ruby_2_6_0

CI against Ruby 2.6.0

Author: koic

.circleci/config.yml

@@ -35,6 +35,9 @@ workflows:
       - rake_default:
           name: Ruby 2.5
           image: circleci/ruby:2.5
+      - rake_default:
+          name: Ruby 2.6
+          image: circleci/ruby:2.6
       - rake_default:
           name: Ruby HEAD
           image: rubocophq/circleci-ruby-snapshot:latest # Nightly snapshot build