Use bundler-cache instead of bundle install

Author: hsbt

.github/workflows/test.yml

@@ -37,11 +37,7 @@ jobs:
       uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
-    - name: Install
-      id: bundle
-      run: |
-        bundle install
-      if: "matrix.ruby <= '2.1'"
+        bundler-cache: true
     - name: Run test
       run: bundle exec rake test
     - id: build