title: Why does Rails fails to boot with "Expected to find a manifest file in `app/assets/config/manifest.js` (Sprockets::Railtie::ManifestNeededError)"?
body: <p>After <code>bundle update</code> my Rails app fails to boot with:</p>
  
  <pre><code>Expected to find a manifest file in `app/assets/config/manifest.js` (Sprockets::Railtie::ManifestNeededError)
  </code></pre>