chore: Update CI workflow to use bundler cache and bundle exec rake for running tests

Author: hahwul

.github/workflows/ci.yml

@@ -23,8 +23,6 @@ jobs:
       uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby-version }}
-        bundler-cache: false # runs 'bundle install' and caches installed gems automatically
-    - name: Install dependencies
-      run: gem install rspec
+        bundler-cache: true # runs 'bundle install' and caches installed gems automatically
     - name: Run tests
-      run: rspec
+      run: bundle exec rake