ci.yml in rubocop-sorbet-0.6.8

- old
+ new

@@ -9,10 +9,10 @@
   build:
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ 2.5, 2.6, 2.7, 3.0 ]
+        ruby: [ '2.5', '2.6', '2.7', '3.0', '3.1' ]
     name: Test Ruby ${{ matrix.ruby }}
     steps:
       - uses: actions/checkout@v2
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1