Add Ruby head version to CI matrix

Author: kojix2

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ jobs:
     strategy:
       matrix:
         os: ["ubuntu", "macos"]
-        ruby: ["2.6", "2.7", "3.0", "3.1", "3.2", "3.3", "3.4"]
+        ruby: ["2.6", "2.7", "3.0", "3.1", "3.2", "3.3", "3.4", "head"]
     steps:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1