This terrible thing works

Gemfile

source 'http://rubygems.org'

gem 'bundler', '~> 1.0.0'
gem 'decent_exposure', '~> 1.0.0.rc1'
gem 'devise', '~> 1.1.2'
gem 'haml'
gem 'hassle', :git => 'git://github.com/Papipo/hassle.git'
gem 'pg'
gem 'rails', '3.0.0'

dev_gemfile = File.read('Gemfile.development')
eval(dev_gemfile) unless ENV.any?{|key| key =~ /^HEROKU_/}

Gemfile.development

group :development, :test do
  gem 'ruby-debug-base19', '0.11.24'
  gem 'ruby-debug19', '0.11.6'
  gem 'factory_girl_rails'
  gem 'rails3-generators'
  gem 'thin'
  gem 'pickler', '~> 0.1.7'
end

group :test do
  gem 'capybara', '~> 0.3.9'
  gem 'cucumber-rails', '0.3.2'
  gem 'launchy'
  gem 'rspec', '2.0.0.beta.20'
  gem 'rspec-core', '2.0.0.beta.20'
  gem 'rspec-mocks', '2.0.0.beta.20'
  gem 'rspec-expectations', '2.0.0.beta.20'
  gem 'rspec-rails', '2.0.0.beta.20'
  gem 'shoulda', '2.11.3'
end

I couldn't find a way to do --without on Heroku either.