upgrade JRuby, libraries, Maven plugins and RubyGems; fix upstream test script

- upgrade JRuby to 1.7.16.1
- upgrade Maven plugins
- upgrade libraries (slf4j, jcommander, guava, jsoup)
- upgrade to Asciidoctor 1.5.2
- upgrade dependent gems (haml, tilt, slim)
- fix upstream test script
- add JDK 8 JVMs to the Travis CI test matrix