Enable bundler cache in GitHub Actions (#450)

Use ruby/setup-ruby's bundler-cache option to cache installed gems
keyed by Gemfile.lock, reducing CI time spent on bundle install.

The option runs bundle install automatically, so the explicit
Bundle install step was removed.

Author: sinsoku

.github/workflows/main.yml

@@ -33,9 +33,7 @@ jobs:
       uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
-    - name: Bundle install
-      run: |
-        bundle install
+        bundler-cache: true
     - name: Run the test suite
       run: |
         bundle exec rake TESTOPT=-v RUBYOPT="${{ matrix.rubyopt }}"