Move to fetching the jruby-bin tarball

This also updates untar() to respect file permissions and skip copying
if the file size and permissions are correct.

The major change in vendoring here is that we don't use jruby-complete
anymore. We now use bin/jruby as the entry point.

This improves the startup time of Logstash on my laptop by approximately
1.5 seconds (~25% speedup of startup time)

Testing `bin/logstash version` speed; 5 runs each:
* Before patch: 7.27 7.55 7.26 7.42 7.47
* After patch: 5.54 5.97 5.67 5.58 5.54