title: django geoposition map doesn't appear in admin
body: <p>I'm using django-geoposition in a project and added it to my admin with a <code>GeopositionField()</code> in my <code>models.py</code>.</p>
  
  <pre><code>class MyClass(models.Model):
      position = GeopositionField()
  </code></pre>
  
  <p>When I access it, it shows the <code>lat, long</code> field, but not the google map.
  There are no errors in the browser console.</p>
  
  <p>I have the application configured correctly in <code>settings.py</code>: <code>INSTALLED_APPS</code> and <code>GEOPOSITION_GOOGLE_MAPS_API_KEY</code>.</p>
  
  <p><strong>Django version==1.11.5</strong></p>
  
  <p>What can cause this?</p>