Try to fix CI for Ruby 2.7

Author: sshaw

.github/workflows/main.yml

@@ -24,5 +24,10 @@ jobs:
           bundler-cache: true
           ruby-version: ${{ matrix.ruby }}
 
+      - if: startsWith(matrix.ruby, '2.7')
+        run: |
+          gem uninstall bundler -a -x || true
+          gem install bundler -v "~> 2.4"
+
       - run: bundle install
       - run: bundle exec rake