ruby-on-rails - rdiscount gem を使用した Heroku のエラー

Question

必要かもしれないと思ったものに rdiscount gem を使用しましたが、アンインストールしました。アプリを Heroku にデプロイしようとすると、次のようになります。

Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.
       /usr/local/bin/ruby extconf.rb
       checking for random()... yes
       checking for srandom()... yes
       checking for rand()... yes
       checking for srand()... yes
       checking size of unsigned long... long
       checking size of unsigned int... int
       no int with size 4
       *** extconf.rb failed ***
       Could not create Makefile due to some reason, probably lack of
       necessary libraries and/or headers.  Check the mkmf.log file for more
       details.  You may need configuration options.
       Provided configuration options:
       --with-opt-dir
       --without-opt-dir
       --with-opt-include
       --without-opt-include=${opt-dir}/include
       --with-opt-lib
       --without-opt-lib=${opt-dir}/lib
       --with-make-prog
       --without-make-prog
       --srcdir=.
       --curdir
       --ruby=/usr/local/bin/ruby
       --with-rdiscount-dir
       --without-rdiscount-dir
       --with-rdiscount-include
       --without-rdiscount-include=${rdiscount-dir}/include
       --with-rdiscount-lib
       --without-rdiscount-lib=${rdiscount-dir}/lib
       Gem files will remain installed in /tmp/build_2dddf5mp2ptjr/vendor/bundle/ruby/1.9.1/gems/rdiscount-2.0.7.1 for inspection.
       Results logged to /tmp/build_2dddf5mp2ptjr/vendor/bundle/ruby/1.9.1/gems/rdiscount-2.0.7.1/ext/gem_make.out
       An error occurred while installing rdiscount (2.0.7.1), and Bundler cannot
       continue.
       Make sure that `gem install rdiscount -v '2.0.7.1'` succeeds before bundling.
 !
 !     Failed to install gems via Bundler.

私は宝石をアンインストールしましたgem unistall rdiscount が、何もしません。次に、herokuエラーごとに再インストールしようとしましたが、まだ何もありません。

Gemfile:

source 'http://rubygems.org'

gem 'rails', '3.2.11'
gem 'bootstrap-sass', '2.3.0'


group :development, :test do
  gem 'sqlite3', '1.3.5'
  gem 'rspec-rails', '2.11.0'
  gem 'guard-rspec', "1.2.1"
  gem 'guard-spork', '1.4.2'
  gem 'spork', "0.9.2"
  gem 'guard-livereload'
end

group :assets do
  gem 'sass-rails',   '3.2.5'
  gem 'coffee-rails', '3.2.2'
  gem 'uglifier', '1.2.3'
end

gem 'jquery-rails', '2.0.2'

group :test do
  gem 'capybara', '1.1.2'
  gem 'rb-fsevent', "0.9.1", :require => false
  gem 'factory_girl_rails', "4.1.0"
end

group :production do
  gem 'pg', '0.12.2'
end