Install bundler instead of trying to upgrade Ruby

Ruby 2.7 is not available yet.

.github/workflows/rubocop.yml

@@ -11,11 +11,14 @@ jobs:
     - name: Set up Ruby 2.6
       uses: actions/setup-ruby@v1
       with:
-        ruby-version: 2.7.x
+        ruby-version: 2.6.x
     - name: Install required package
       run: |
         sudo apt-get update
         sudo apt-get install libmysqlclient-dev libpq-dev libsqlite3-dev libncurses5-dev
+    - name: Install bundler
+      run: |
+        gem install bundler:2.1.2
     - name: Cache gems
       uses: actions/cache@v1
       with: