title: can't install capybara-webkit
body: <p>When I <code>bundle install</code> I get:</p>
  
  <pre><code>Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
  
      /usr/bin/ruby2.1 extconf.rb
  Command 'qmake -spec linux-g++ ' not available
  
  Makefile not found
  
  Gem files will remain installed in /tmp/bundler20141128-11764-1rcu7gh/capybara-w
  ebkit-1.3.0/gems/capybara-webkit-1.3.0 for inspection.
  Results logged to /tmp/bundler20141128-11764-1rcu7gh/capybara-webkit-1.3.0/exten
  sions/x86-linux/2.1.0/capybara-webkit-1.3.0/gem_make.out
  An error occurred while installing capybara-webkit (1.3.0), and Bundler cannot
  continue.
  Make sure that `gem install capybara-webkit -v '1.3.0'` succeeds before
  bundling.
  </code></pre>
  
  <p>Any idea?</p>