CI: Test with Ruby 4.0

Author: bquorning

.github/workflows/actions.yml

@@ -7,12 +7,13 @@ jobs:
     strategy:
       matrix:
         ruby:
-          - 2.7
-          - 3.0
-          - 3.1
-          - 3.2
-          - 3.3
-          - 3.4
+          - "2.7"
+          - "3.0"
+          - "3.1"
+          - "3.2"
+          - "3.3"
+          - "3.4"
+          - "4.0"
     steps:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1