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.
Can we refactor this to:

```
cmd = LogStash::Environment.ruby(LogStash::Environment.bundler_install_command("tools/Gemfile.plugins", LogStash::Environment::PLUGINS_DIR))
```

and add an Environment.ruby() method that returns the command? 

Maybe `ruby` is a bad name in this case, but mostly wanting to reduce copy/paste.