title: Push to heroku fails: revealed dependencies not in the API or the lockfile
body: <p>I can't push to heroku.
  The bundler has found unmet dependencies of nokogiri 1.6.8.1.</p>
  
  <pre><code>$ git push heroku master
   ....
  remote:        The latest bundler is 1.15.4, but you are currently running 1.15.2.
  remote:        To update, run `gem install bundler`
  remote:        Downloading nokogiri-1.6.8.1 revealed dependencies not in the API or the
  remote:        lockfile (mini_portile2 (~&gt; 2.1.0)).
  remote:        Either installing with `--full-index` or running `bundle update nokogiri` should
  remote:        fix the problem.
  remote:  !
  remote:  !     Failed to install gems via Bundler.
  remote:  !
  remote:  !     Push rejected, failed to compile Ruby app.
  remote: 
  remote:  !     Push failed
  remote: Verifying deploy...
  remote: 
  remote: !       Push rejected to digitreco.
  </code></pre>
  
  <p>I am on the Heroku -16 stack. I tried downgrading nokogiri to 1.5.9 in the Gemfile but it resolves to 1.6.0 due to other dependencies.</p>
  
  <p>Any help is appreciated. </p>