Use default rubygems and bundler for CI

Author: pilaf

.github/workflows/ci.yml

@@ -46,8 +46,8 @@ jobs:
       uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
-        rubygems: latest
-        bundler: latest
+        rubygems: default
+        # bundler: latest
         # bundler-cache: true
     - name: Install dependencies
       env: