Bundler on 2.3.5

boot.rb

# add this to the bottom of config/boot.rb, before the line `Rails.boot!`

class Rails::Boot
  def run
    load_initializer
    extend_environment
    Rails::Initializer.run(:set_load_path)
  end

  def extend_environment
    Rails::Initializer.class_eval do
      old_load = instance_method(:load_environment)
      define_method(:load_environment) do
        Bundler.require :default, Rails.env
        old_load.bind(self).call
      end
    end
  end
end

Gemfile

# include at least one source and the rails gem
source :gemcutter
gem "rails", "~> 2.3.5", :require => nil
gem "sqlite3-ruby", :require => "sqlite3"

group :development do
  # bundler requires these gems in development
  gem 'rails-footnotes'
end

group :test do
  # bundler requires these gems while running tests
  gem 'rspec'
end

preinitializer.rb

# this code goes in config/preinitializer.rb, which you should create if it doesn't exist

begin
  # Require the preresolved locked set of gems.
  require File.expand_path('../../.bundle/environment', __FILE__)
rescue LoadError
  # Fallback on doing the resolve at runtime.
  require "rubygems"
  require "bundler"
  Bundler.setup
end