Add ruby 3.1 to the build matrix

Author: jmortlock

.github/workflows/ruby.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ["2.6", "2.7", "3.0"]
+        ruby: ["2.6", "2.7", "3.0", "3.1"]
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2