Use bundler-cache option of ruby/setup-ruby

ruby/setup-ruby has caching built in and is guaranteed to be correct.

.github/workflows/main.yml

@@ -55,19 +55,9 @@ jobs:
           PHP_TZ: "US/Eastern"
     steps:
     - uses: actions/checkout@v2
-    - uses: ruby/setup-ruby@v1 # actions/setup-ruby@v1
+    - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
-    - uses: actions/cache@v2
-      with:
-        path: vendor/bundle
-        key: ${{ matrix.ruby }}-gems-${{ hashFiles('**/Gemfile.lock') }}
-        restore-keys: |
-          ${{ matrix.ruby }}-gems-
-    - name: Bundle install
-      run: |
-        gem list --silent -i bundler -v '~> 2' || gem install bundler -v '~> 2' --no-document
-        bundle config path vendor/bundle
-        bundle install --without development --jobs 4 --retry 3
+        bundler-cache: true
     - name: Run tests
       run: bundle exec rspec ./spec/*