ci: let's try to build using older jruby (9.4)

Author: kares

.github/workflows/ci.yml

@@ -28,6 +28,7 @@ jobs:
         include:
           - { os: windows-latest , ruby: mingw }
           - { os: windows-latest , ruby: mswin }
+          - { os: ubuntu-latest , ruby: jruby-9.4 }
         exclude:
           - { os: macos-latest , ruby: 2.4 }
           - { os: macos-latest , ruby: 2.5 }