run CI against latest stable ruby instead of head

Author: lsylvester

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ ubuntu, macos, windows ]
-        ruby: [ '2.5', ruby-head ]
+        ruby: [ '2.5', ruby ]
     runs-on: ${{ matrix.os }}-latest
     env:
       PGSERVICE: postgres