Add Ruby 3.1 to the CI matrix (#365)

Merge pull request 365

Author: mishina2228

.github/workflows/ruby.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['2.6', '2.7', '3.0']
+        ruby-version: ['2.6', '2.7', '3.0', '3.1']
 
     steps:
     - uses: actions/checkout@v2