title: Invalid User resolver, callable or UserResolver FQCN expected error [Laravel Auditing 4.1]
body: <p>I am using Laravel 5.4 and Laravel Auditing 4.1. I want to record the price changes on Variant model, I installed the Auditing 4.1 package. See my code,</p>
  
  <p><strong>config/audit.php</strong></p>
  
  <pre><code>'implementation' =&gt; OwenIt\Auditing\Models\Audit::class,
  
  'user' =&gt; [
      'primary_key' =&gt; 'id',
      'foreign_key' =&gt; 'variant_id',
      'model'       =&gt; App\Variant::class,
      'resolver'    =&gt; App\Variant::class,
  ],
  </code></pre>
  
  <p><strong>app/Models/Variant.php</strong></p>
  
  <pre><code>use OwenIt\Auditing\Auditable;
  use OwenIt\Auditing\Contracts\Auditable as AuditableContract;
  
  class Variant extends BaseModel implements AuditableContract{
  
     use Auditable;
   --------Code here----------------
  }
  </code></pre>
  
  <p>Using this code when I try to insert data to database, This error will come,</p>
  
  <pre><code> UnexpectedValueException
   Invalid User resolver, callable or UserResolver FQCN expected
  </code></pre>
  
  <p>How can I fix this error and record my changes on <code>audits</code> table?</p>