Add option to use a local jruby version instead of the vendored one

If we want to test against custom Jruby versions we need to ensure we use the correct jruby binary
Like what we can do with USE_RUBY=1 ./bin/logstash to use the local ruby binary we can do the same with the rake bootstrap job
you can just do 

`if ENV["USE_RUBY"] != "1" && (RUBY_ENGINE != "jruby" or Gem.ruby !~ /vendor\/jruby\/bin\/jruby/)`