test ruby 2.7.8 only with 3.1.0 selected in the matrix

Author: jvmvik

.github/workflows/ci.yml

@@ -33,9 +33,13 @@ jobs:
         run: bundle exec rake oobt
       - name: Run example with ruby 2.7.8
         uses: ruby/setup-ruby@v1
+        if: matrix.ruby-version == '3.1.0'
         with:
           ruby-version: '2.7.8'
       - name: Install and run demo 2.7.8
+        if: matrix.ruby-version == '3.1.0'
         env:
           SERPAPI_KEY: ${{ secrets.SERPAPI_KEY }}
-        run: gem install ./serpapi-*.gem && ruby demo/demo.rb
+        run: |
+          gem install ./serpapi-*.gem
+          ruby demo/demo.rb