config

BUNDLE_WITHOUT                     => development:test

Gemfile

group :development, :test do  
  if RUBY_PLATFORM =~ /darwin/i
    gem 'growl', '1.0.3'
    gem 'rb-fsevent', '0.4.3.1'
  end
end

gistfile1.txt

-----> Heroku receiving push
-----> Ruby/Rails app detected
-----> Installing dependencies using Bundler version 1.1.pre.8
       Running: bundle install --without development:test --path vendor/bundle --deployment
       You are trying to install in deployment mode after changing
       your Gemfile. Run `bundle install` elsewhere and add the
       updated Gemfile.lock to version control.
       You have deleted from the Gemfile:
       * growl (= 1.0.3)
       * rb-fsevent (= 0.4.3.1)
 !
 !     Failed to install gems via Bundler.
 !
 !     Heroku push rejected, failed to compile Ruby/rails app