Bumped jruby to 1.7.13

JRuby 1.7.13 contains lots of improvements compared to 1.7.11, including this very important one: https://github.com/jruby/jruby/pull/1546

FYI we notice a solid 25% speed gain when bootstraping our JRuby Vert.x applications when we manually replace JRuby with version 1.7.13 in the fat-jar we run.