Test Ruby 3.1 on CI

Author: splattael

.github/workflows/ruby.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['2.6', '2.7', '3.0', head, jruby, truffleruby]
+        ruby-version: ['2.6', '2.7', '3.0', '3.1', head, jruby, truffleruby]
 
     steps:
     - uses: actions/checkout@v2