Add JRuby to CI

Author: headius

.github/workflows/test.yml

@@ -7,7 +7,7 @@ jobs:
     name: build (${{ matrix.ruby }} / ${{ matrix.os }})
     strategy:
       matrix:
-        ruby: [ 2.7, 2.6, 2.5, head ]
+        ruby: [ 2.7, 2.6, 2.5, head, jruby, jruby-head ]
         os: [ ubuntu-latest, macos-latest, windows-latest ]
         exclude:
           - { os: macos-latest, ruby: '2.5' }