bundler as version conflict resolver (2)

And to switch in between Ruby version[0],
usually I'll use `/path/to/ruby -S ruby_script_file`.
In bundler, it's simply:

bundle exec /usr/bin/ruby -S unicorn_rails

This way, unicorn_rails is executed with system Ruby,
which is versioned 1.8.7, in a bundler's environment.

[0] Sorry, rvm is not working for my fish shell on Mac OS 10.6