[#18] Sanitize the environment of the new Ruby process.

The new Ruby exec'd by RVM can be affected by existing environment variables.  Specifically, a
current Ruby process running with bundler may have "RUBYOPT=-rbundler/setup", which will cause
breakage if the bundle has not yet been installed a new Ruby.

Remove the variables to ensure independence and expected behaviour of the new Ruby.  Unset one at a
time for readbility.