title: laravel carbon isoFormat method does not exist
body: <p>I tried to use the Laravel carbon isoFormat method.  I received the error Method isoFormat does not exist.  My Laravel implementation is 5.7.  I tried composer update, and verified that it updated nesbot/carbon.</p>
  
  <p>I copied and pasted code from Mr. Nesbot's manual to see if his code would work.</p>
  
  <pre><code>$mutable = Carbon::now();
  var_dump($mutable-&gt;isoFormat('dddd D'));
  </code></pre>
  
  <p>Mr. Nesbot's code produces the same error.</p>
  
  <p>How do I resolve this error, please?</p>