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.
hmmm. this `GEM_PATH` array is actually different in `plugin.rake` and `vendor.rake` - in `vendor.rake` it does not include the plugins path, but actually the plugins path does not exist yet so I guess it would be harmless to include it.