title: Exclude hoe gem dependency from Heroku
body: <p>I am trying to push code to Heroku and it seems like the system gem "hoe" is somehow included by some other gem (geokit was one of them but no luck even after removing that gem from Gemfile). This is causing Heroku to fail because it cannot update system gems. how to solve this issue?</p>
  
  <pre><code>Installing geokit (1.5.0) 
     Installing geonames (0.2.2) 
     Installing hoe (2.9.1) /usr/ruby1.8.7/lib/ruby/site_ruby/1.8/rubygems/installer.rb:170:in `install': hoe requires RubyGems version &gt;= 1.4. Try 'gem update --system' to update RubyGems itself. (Gem::InstallError)
      from /usr/ruby1.8.7/lib/ruby/gems/1.8/gems/bundler-1.0.7/lib/bundler/source.rb:95:in `install'
      from /usr/ruby1.8.7/lib/ruby/gems/1.8/gems/bundler-1.0.7/lib/bundler/installer.rb:55:in `run'
      from /usr/ruby1.8.7/lib/ruby/gems/1.8/gems/bundler-1.0.7/lib/bundler/spec_set.rb:12:in `each'
      from /usr/ruby1.8.7/lib/ruby/gems/1.8/gems/bundler-1.0.7/lib/bundler/spec_set.rb:12:in `each'
      from /usr/ruby1.8.7/lib/ruby/gems/1.8/gems/bundler-1.0.7/lib/bundler/installer.rb:44:in `run'
      from /usr/ruby1.8.7/lib/ruby/gems/1.8/gems/bundler-1.0.7/lib/bundler/installer.rb:8:in `install'
      from /usr/ruby1.8.7/lib/ruby/gems/1.8/gems/bundler-1.0.7/lib/bundler/cli.rb:225:in `install'
      from /usr/ruby1.8.7/lib/ruby/gems/1.8/gems/bundler-1.0.7/lib/bundler/vendor/thor/task.rb:22:in `send'
      from /usr/ruby1.8.7/lib/ruby/gems/1.8/gems/bundler-1.0.7/lib/bundler/vendor/thor/task.rb:22:in `run'
      from /usr/ruby1.8.7/lib/ruby/gems/1.8/gems/bundler-1.0.7/lib/bundler/vendor/thor/invocation.rb:118:in `invoke_task'
      from /usr/ruby1.8.7/lib/ruby/gems/1.8/gems/bundler-1.0.7/lib/bundler/vendor/thor.rb:246:in `dispatch'
      from /usr/ruby1.8.7/lib/ruby/gems/1.8/gems/bundler-1.0.7/lib/bundler/vendor/thor/base.rb:389:in `start'
      from /usr/ruby1.8.7/lib/ruby/gems/1.8/gems/bundler-1.0.7/bin/bundle:13
      from /usr/ruby1.8.7/bin/bundle:19:in `load'
      from /usr/ruby1.8.7/bin/bundle:19
     FAILED: http://devcenter.heroku.com/articles/bundler
  </code></pre>
  
  <p>!     Heroku push rejected, failed to install gems via Bundler</p>