CI: add Ruby 3.4 and 4.0 (#160)

Signed-off-by: Shizuo Fujita <[email protected]>

Author: Watson1978

.github/workflows/linux.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '3.3', '3.2', '3.1', '3.0', '2.7' ]
+        ruby: [ '4.0', '3.4', '3.3', '3.2', '3.1', '3.0', '2.7' ]
         postgres: [ '16', '15', '14', '13', '12' ]
         os:
           - ubuntu-latest