language: ruby
rvm:
  # https://www.ruby-lang.org/en/downloads/branches/
  - 2.6
  - 2.5
  - 2.4
  # macOS system Ruby is Ruby 2.3
  - 2.3
env:
  matrix:
    - OJ=0
    - OJ=1
branches:
  only:
    - master
before_script:
  - if [[ $OJ -eq 1 ]]; then gem install oj; fi