Test mruby-engine on ruby 4.0 and head

Author: rwstauner

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '3.2', '3.3', '3.4' ]
+        ruby: [ '3.2', '3.3', '3.4', '4.0', 'head' ]
     env:
       W_ERROR: '1'
     steps: