Use Bundler.clean_system under a Bundler environment when invoking RhoRu...

In order to address Issue #180, I wrote a patch for the Rakefile.

The main problem is that the RUBYOPT environment variable set by Bundler is propagated to the custom Ruby in Rhodes. In order to fix it, we can use Bundler.clean_system, which resets the environment variable before invoking a child process.