Add jruby-head to macos CI builds

Author: headius

.github/workflows/macos.yml

@@ -9,7 +9,7 @@ jobs:
     with:
       engine: cruby
       min_version: 2.7
-      versions: '["debug"]'
+      versions: '["debug", "jruby-head"]'
 
   build:
     needs: ruby-versions