Use ruby/setup-ruby action to use Ruby 2.7

https://github.com/marketplace/actions/setup-ruby-jruby-and-truffleruby

Because Ruby 2.7 already installs bundler by default, no need to install
bundler separately.

Author: yahonda

.github/workflows/rubocop.yml

@@ -8,17 +8,14 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Ruby 2.6
-      uses: actions/setup-ruby@v1
+    - name: Set up Ruby 2.7
+      uses: ruby/setup-ruby@v1
       with:
-        ruby-version: 2.6.x
+        ruby-version: 2.7
     - name: Install required package
       run: |
         sudo apt-get update
         sudo apt-get -y install libmysqlclient-dev libpq-dev libsqlite3-dev libncurses5-dev
-    - name: Install bundler
-      run: |
-        gem install bundler:2.1.2 --no-document
     - name: Cache gems
       uses: actions/cache@v1
       with: