gem paths fixes and refactor

this is based on @ph PR #2200.
this has been tested using JRuby as the default interpreter with
```sh
rake bootstrap
rake plugin:install-defaults
./bin/logstash rspec vendor/plugins/jruby/1.9/gems/logstash-*/spec/{input,output,filter,codec}s/*_spec.rb --order rand
```

this has not been tested with MRI as the default interpreter.
this whole setup for the Bundler::CLI call is a hack, carefully crafter after figuring out the particular Bundler logic, obviously not meant to be used as an api. we will need to re-evaluate this strategy and/or push/contribute for a better Bundler API :P