Use RUBYLIB instead of RUBYOPT to add bundle lib path to ruby $LOAD_PATH.

As discussed in #2529, resolves #886.

This allows `bundle exec` to run when the bundler gem is located at a path that includes spaces (which is pretty common in windows environments).
We have tested this patch on windows (with JRuby) and use it already in production. It would be super awesome to get this merged in, so we no longer need to monkey patch bundler.