Strip support for 1.8 and 1.9 from JRuby

With the aim to reduce unnecessary space, this PR strips support from the vendorized JRuby for 1.8 and 2.0 libs. It also makes sure that logstash is run under the 1.9 mode.

Fits the point number four of #2137 

 
once you have this add the `./vendor/jruby/lib/ruby/.+?/rdoc` too, pls (5. bullet in the slim down list)