title: Model does not create a verion after deleting a foreign key model object (django-reversion)
body: <p>I have 2 classes:</p>
  
  <pre><code>class ContactInternal(models.Model):
      name = models.CharField(max_length=80)
  SysAppApp = apps.get_app_config('SysApp')
  SysAppModel = SysAppApp.models
  reversion.register(ContactInternal, follow=["fk_contactinternal_sysapp"])
  reversion.register(SysAppModel)
  
  # SysApp from another app called 'SysApp'
  class SysApp(models.Model):
      name = models.CharField(max_length=80)
      internalcontact = models.ForeignKey(ContactInternal, related_name='fk_contactinternal_sysapp', null=True, verbose_name="Internal Contact",blank=True,on_delete=models.SET_NULL)
  </code></pre>
  
  <p>It is working fine for most of the case. e.g. when I make a change to ContactInternal, a new version is made to SysApp. Pretty happy with how it works.</p>
  
  <p>However if I delete a record linked from <code>SysApp.internalcontact</code>, no version is created for SysApp.</p>
  
  <p>Actually all I want to achieve is to get a timestamp of the last modified date of SysApp and by which user(don't need to rollback). I want to know </p>
  
  <pre><code>'When Peter delete a John from ContactInternal on 26th Feb, it will show Peter updated SysApp on 26th Feb'
  </code></pre>
  
  <p>How can I do it with <code>django-reversion</code>? Or is there another way to achieve this?</p>
  
  <ul>
  <li>Note: There is another field in SysApp which is M2M I would like to achieve the same result too.</li>
  </ul>