🔧 Set bundler version for ancient ruby 2.7

Author: pboling

.github/workflows/ancient.yml

@@ -41,6 +41,7 @@ jobs:
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: "${{ matrix.ruby }}"
+          bundler: "2.4.22"
           bundler-cache: true # runs 'bundle install' and caches installed gems automatically
       - name: Run tests
         run: bundle exec rake test