ci: Use bundler-cache instead of manual Bundler install

Let ruby/setup-ruby manage Bundler and dependency installation.
This avoids the manual Bundler install step and handles version
differences more reliably.

Author: tokengeek

.github/workflows/ruby.yml

@@ -32,9 +32,6 @@ jobs:
       uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
-    - name: Install Bundler
-      run: gem install bundler -v $(grep -A 1 "BUNDLED WITH" Gemfile.lock | tail -n 1 | awk '{print $1}')
-    - name: Install dependencies
-      run: bundle install
+        bundler-cache: true
     - name: Run tests
       run: bundle exec rake