bundler for rails 2.3.9 on heroku

It seems... heroku is not the same environment as mine.
I need to patch the following:

1. add bundler and i18n to Gemfile
2. add require 'bundler/setup' in boot.rb (i.e. in load_gems)
3. remove all config.gems in environment.rb

Not sure why, but now it is working.