CI against JRuby at GitHub Actions

- CI against JRuby 9.2 as `jruby` and `jruby 9.3.0.0-SNAPSHOT` as
  `jruby-head`
- Workaround for bundle inline added to `remove Gemfile.lock`
  Refer ruby/rubygems#3796

Author: yahonda

.github/workflows/test.yml

@@ -15,7 +15,9 @@ jobs:
           2.6,
           2.5,
           ruby-head,
-          ruby-debug
+          ruby-debug,
+          jruby,
+          jruby-head
         ]
     env:
       ORACLE_HOME: /usr/lib/oracle/18.5/client64
@@ -64,6 +66,9 @@ jobs:
     - name: Run RSpec
       run: |
         bundle exec rspec
+    - name: Workaround jruby-head failure by removing Gemfile.lock
+      run: |
+        rm Gemfile.lock
     - name: Run bug report templates
       run: |
         ruby guides/bug_report_templates/active_record_gem.rb