Add Ruby 3.1 to the CI matrix

Author: petergoldstein

.github/workflows/ruby.yml

@@ -17,7 +17,7 @@ jobs:
 
     strategy:
       matrix:
-        ruby: [2.5, 2.6, 2.7, "3.0"]
+        ruby: [2.5, 2.6, 2.7, "3.0", 3.1]
 
     steps:
     - uses: actions/checkout@v2