Bump the jar plugin dependency up to 0.1.8+
Bump the jar plugin dependency up to 0.1.8+

Fixes #18

MisumiRize/jruby-gradle-plugin,jamescway/jruby-gradle-plugin,jamescway/jruby-gradle-plugin,raelik/jruby-gradle-plugin,raelik/jruby-gradle-plugin,MisumiRize/jruby-gradle-plugin,MisumiRize/jruby-gradle-plugin,jamescway/jruby-gradle-plugin,raelik/jruby-gradle-plugin