Add Ruby 3.1 to the CI matrix (#149)

Author: petergoldstein

Diff for: .github/workflows/ci.yml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [2.7, "3.0"]
+        ruby: [2.7, "3.0", 3.1]
 
     steps:
       - uses: actions/checkout@v2