Let setup-ruby choose appropriate version of bundler

instead of manually installing

Author: amatsuda

.github/workflows/ruby.yml

@@ -7,9 +7,6 @@ jobs:
     name: Ruby specs
     runs-on: ${{ matrix.os }}
     continue-on-error: ${{ endsWith(matrix.ruby, 'head') || matrix.ruby == 'debug' || matrix.experimental }}
-    env:
-      BUNDLE_JOBS: 4
-      BUNDLE_RETRY: 3
     strategy:
       fail-fast: false
       matrix:
@@ -34,25 +31,15 @@ jobs:
             gemfile: gemfiles/Gemfile-rails-edge
             experimental: true
 
+    env:
+      BUNDLE_GEMFILE: ${{ matrix.gemfile }}
     steps:
       - uses: actions/checkout@v2
-      - uses: actions/cache@v2
-        with:
-          path: /home/runner/bundle
-          key: bundle-use-ruby-${{ matrix.ruby }}-${{ matrix.gemfile }}-gems-${{ hashFiles(matrix.gemfile) }}-${{ hashFiles('**/*.gemspec') }}
-          restore-keys: |
-            bundle-use-ruby-${{ matrix.ruby }}-${{ matrix.gemfile }}-gems-
 
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}
-
-      - name: Bundle install
-        run: |
-          gem install bundler -v 2.1.4
-          bundle config path /home/runner/bundle
-          bundle config --global gemfile ${{ matrix.gemfile }}
-          bundle install --jobs 4 --retry 3
+          bundler-cache: true
 
       - name: Ruby specs
         run: bundle exec rake test