[Quicky] Github Action: cache Ruby's gems

[Quicky] Github Action: cache Ruby's gems

You just need bundler-cache: true

# .github/workflows/my_very_special_job.yml
jobs:
  my_very_special_job:
    steps:
      - name: Setup Ruby
        uses: ruby/setup-ruby@v1
        with:
          bundler-cache: true    ###### That's the line