Add Ruby 4.0 preview 2 to test matrix

Author: vcsjones

.github/workflows/ruby.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [ '3.1', '3.2', '3.3', '3.4' ]
+        ruby: [ '3.1', '3.2', '3.3', '3.4', '4.0.0-preview2' ]
 
     steps:
     - uses: actions/checkout@v4