Add extra bundle install step in Github CI

Author: drwl

.github/workflows/ci.yml

@@ -29,6 +29,7 @@ jobs:
           gem update --system
           gem update bundler
           bundle install --jobs 4 --retry 3
+          BUNDLE_GEMFILE=spec/integration/rails_6.0.2.1/Gemfile bundle install --jobs 4 --retry 3
 
       - name: Test
         run: bundle exec rubocop && bundle exec rspec