Make necessary changes to build with bundler 2.x, JRuby 9.3 and Java 11+

[jsvd]

• The build still uses Bundler 1.x and JRuby 9.2 (jar-dependencies uses jruby 9.0).
• mvn package seems to fail on java 11+
• I also hit https://github.com/torquebox/jruby9-maven-plugins/issues/5

Stretch goal: add CI for testing

[kares]

some work in progress, has been around at: https://github.com/guyboertje/jrjackson/pull/85

[jsvd]

fixed by #85 and #98