Add JRuby to CI

Author: headius

.github/workflows/ubuntu.yml

@@ -12,7 +12,7 @@ jobs:
         os:
         - ubuntu-20.04
         - ubuntu-18.04
-        ruby: [ '3.0', '2.7', '2.6', 'debug', 'head' ]
+        ruby: [ '3.0', '2.7', '2.6', 'debug', 'head', 'jruby-head' ]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Ruby