✅🚧 Run CI with JRuby, too

Using jruby-head to get some bugfixes.

Author: nevans

.github/workflows/test.yml

@@ -22,6 +22,7 @@ jobs:
         experimental: [false]
         include:
           - { ruby: truffleruby, os: ubuntu-latest,  experimental: true }
+          - { ruby: jruby-head,  os: ubuntu-latest,  experimental: true }
     runs-on: ${{ matrix.os }}
     continue-on-error: ${{ matrix.experimental }}
     timeout-minutes: 15