Add Ruby 4.0 to CI test matrix (#15)

Author: dduugg

.github/workflows/ci.yml

@@ -22,6 +22,7 @@ jobs:
           - 3.2
           - 3.3
           - 3.4
+          - 4.0
     env:
       BUNDLE_GEMFILE: Gemfile
     name: "Run tests: Ruby ${{ matrix.ruby }}"