title: Why do I get `Could not find 'bundler' (1.17.3)` after upgrading Ruby?
body: <p>After upgrading from Ruby 2.3 to a newer version and then doing a <code>bundle install</code> I get the error <code>Could not find 'bundler'</code>.</p>
  
  <p>This happens even after doing <code>gem install bundler</code>.</p>