Pin Ruby 3.1 testing to 3.1.4

Pin to working version while under investigation as import_fragments testcase fails.
See issue #2800

Seems to be same crash as Ruby 3.2.5, see fab27c67ccb66f40001048a6ed5c2927d57b13dc
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d088bdf..3069bf2 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -185,7 +185,7 @@
           VER: '3.0'
           CPPSTD: c++11
         - SWIGLANG: ruby
-          VER: '3.1'
+          VER: '3.1.4' # import_fragments testcase started to fail on upgrade to 3.1.5, see https://github.com/swig/swig/issues/2800
           CPPSTD: c++11
         - SWIGLANG: ruby
           CPPSTD: c++11