chore: Add "head" to GitHub actions Ruby matrix

Ruby 4.0 is expected to be released later this year so this is getting
ready for that.

Author: tokengeek

.github/workflows/ruby.yml

@@ -21,7 +21,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ["2.7", "3.0", "3.1", "3.2", "3.3"]
+        ruby: ["2.7", "3.0", "3.1", "3.2", "3.3", "head"]
     env:
       DEBUG: "true"
     steps: