upgrade JRuby and JRuby plugin

- upgrade JRuby to 1.7.17
- upgrade JRuby plugin to 0.1.10
- tweak Gradle build to allow tests to be run on JDK6
- reenable openjdk6 in build matrix
Merge pull request #241 from mojavelinux/jruby-upgrade

upgrade JRuby and JRuby plugin