gem bundling refactor

- gem bundling now uses the standalone mechanism to generate a setup.rb file which add each individual gem into the load path. this considerably speeds up the logstash boot time. my tests shows it almost cut load time in half. 
- got rid of the custom Gemfile specs loading which did not work when trying to use a local unpublished gem.

after applying this you must:
```sh
make vendor-jruby
bin/logstash deps
```

this is an issue with jar-dependencies 0.0.9, I am not sure why you get this exception - this is why I forced  jar-dependencies 0.1.2 in gembag. I see that I forgot to force version in the gemspec too which shouldn't be a problem since it should pick up what was installed by the gembag. maybe `rm -rf vendor/bundle/jruby` to start clean?