language: ruby
before_install:
  - gem install bundler

matrix:
  include:
    - rvm: 2.5.3 # Pre-installed Ruby version
      script: bundle exec rake rubocop # ONLY lint once, first
    - rvm: 2.3.5
    - rvm: 2.4.6
    - rvm: 2.5.5
    - rvm: 2.6.3
    - rvm: jruby-9.1.8.0
      env:
        - JRUBY_OPTS="--debug"
      jdk: openjdk8
    - rvm: jruby-9.2.8.0
      env:
        - JRUBY_OPTS="--debug"

language: ruby

matrix:
  include:
    - rvm: 2.5.3 # Pre-installed Ruby version
      script: bundle exec rake rubocop # ONLY lint once, first
    - rvm: 2.3.5
    - rvm: 2.4.6
    - rvm: 2.5.5
    - rvm: 2.6.3
    - rvm: jruby-9.1.8.0
      env:
        - JRUBY_OPTS="--debug"
      jdk: openjdk8
    - rvm: jruby-9.2.8.0
      env:
        - JRUBY_OPTS="--debug"

Drop the gem install bundler step
CI: Drop the gem install bundler step