CI against Ruby 2.6.0

Ruby 2.6.0 has been released and this Ruby version is available on CircleCI.

- https://www.ruby-lang.org/en/news/2018/12/25/ruby-2-6-0-released/
- https://hub.docker.com/r/circleci/ruby/tags/

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