title: Bundler stops at gem json install
body: <p>I tried to install susy2 /sass / compass with bundler.
  My Gemfile:</p>
  
  <pre><code>source "https://rubygems.org"
  gem 'sass', '~&gt; 3.3.2'
  gem 'susy', '~&gt; 2.1.1'
  gem 'json', '~&gt; 1.8.1'
  gem 'compass', '~&gt; 1.0.0.alpha.19'
  </code></pre>
  
  <p>bundle install shows the error</p>
  
  <pre><code>Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.
  
          /usr/bin/ruby1.9.1 extconf.rb 
  /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
      from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
      from extconf.rb:1:in `&lt;main&gt;'
  
  Gem files will remain installed in /home/bernie/.bundler/tmp/6210/gems/json-1.8.1 for inspection.
  Results logged to /home/bernie/.bundler/tmp/6210/gems/json-1.8.1/ext/json/ext/generator/gem_make.out
  An error occurred while installing json (1.8.1), and Bundler cannot continue.
  Make sure that `gem install json -v '1.8.1'` succeeds before bundling.
  </code></pre>
  
  <p>gem list shows:</p>
  
  <pre><code>bundler (1.5.1)
  celluloid (0.15.2)
  chunky_png (1.3.0)
  compass (0.12.4)
  compass-core (1.0.0.alpha.19)
  compass-import-once (1.0.4)
  fssm (0.2.10)
  multi_json (1.9.2)
  permutation (0.1.8)
  rake (10.2.0)
  rb-fsevent (0.9.4)
  sass (3.3.4, 3.2.18)
  susy (2.1.1)
  timers (1.1.0)
  </code></pre>
  
  <p>Any Ideas?</p>