CI: Test with Ruby 4.0

Author: bquorning

.github/workflows/ruby.yml

@@ -26,7 +26,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['2.7', '3.0', '3.1', '3.2', '3.3', '3.4', 'ruby-head']
+        ruby-version: ['2.7', '3.0', '3.1', '3.2', '3.3', '3.4', '4.0', 'ruby-head']
 
     steps:
     - uses: actions/checkout@v6