CI: Use bundler-cache: true

Author: olleolleolle

.github/workflows/test.yml

@@ -16,9 +16,6 @@ jobs:
       uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
-    - name: Install dependencies
-      run: |
-        gem install bundler --no-document
-        bundle install
+        bundler-cache: true # 'bundle install' and cache
     - name: Run test
-      run: rake test
+      run: bundle exec rake test