Fix issues with running Ruby 2.0.0 and using Heroku Toolbelt

The issue was that bundle sets ENV['RUBYOPT'] after it has been
loaded, which causes any subprocesses that get spawned
to be confused. The Heroku Toolbelt installs with its own Ruby 1.9.3
binary that lives in /usr/local/heroku/ruby/bin/ruby, so when the
'heroku' script is executed with Ruby 2.0 files in the include path
from bundler, it freaks out.

If you spawn the processes using the incantation
"Bundler.with_clean_env", the RUBYOPT will be temporarily restored.

See for more info:
http://pivotallabs.com/service-oriented-foreman/

We wrote a cucumber test (shell_execution.feature) that exposes
the problem, but it requires that you have the Heroku Toolbelt
installed (not the gem) as well as RVM and Ruby 2.0.

This should fix #151, #150, #146, #138, #133, and #118