title: No such file to load -- json (LoadError)
body: <p>Everything was working fine on this app, and now today I get this when I try to push to Heroku:</p>
  
  <pre><code>Counting objects: 28, done.
  Delta compression using up to 4 threads.
  Compressing objects: 100% (22/22), done.
  Writing objects: 100% (22/22), 2.20 KiB, done.
  Total 22 (delta 14), reused 0 (delta 0)
  /app/slug-compiler/lib/slug.rb:12:in `require': no such file to load -- json (LoadError)
      from /app/slug-compiler/lib/slug.rb:12
      from /app/slug-compiler/bin/slugc:14:in `require'
      from /app/slug-compiler/bin/slugc:14
  To git@heroku.com:cold-night-9597.git
  ! [remote rejected] master -&gt; master (pre-receive hook declined)
   error: failed to push some refs to 'git@heroku.com:server-name-1234.git'
  </code></pre>
  
  <p>Very strange - couldn't find anything on this error anywhere - and its running fine locally...</p>