Add Ruby 3.1 to the CI matrix

Author: petergoldstein

.github/workflows/ruby.yml

@@ -28,7 +28,7 @@ jobs:
 
     strategy:
       matrix:
-        ruby-version: ['2.7', '3.0']
+        ruby-version: ['2.7', '3.0', '3.1']
 
     steps:
     - uses: actions/checkout@v2