Pin bundler to 2.4.22 for Ruby 2.7 compatibility in CI

Author: jaredbrook

.github/workflows/build-gem.yml

@@ -21,7 +21,7 @@ jobs:
 
     - name: rspec
       run: |
-        gem install bundler
+        gem install bundler -v 2.4.22
         bundle install
         bundle exec rspec