title: assertTemplateUsed with django_jinja tries to match full path
body: <p>I've recently upgraded to Django 1.8, replacing the <code>jingo</code> adapter with <code>django_jinja</code> for working with Jinja2 templates. I have a few tests that look like this:</p>
  
  <pre><code>self.assertTemplateUsed(response, 'staff/update_record.jinja')
  </code></pre>
  
  <p>These used to work, but now they fail because it seems to be comparing my string with the full path of the template rather than relative to the template directory:</p>
  
  <pre><code>AssertionError: Template 'staff/update_record.jinja' was not a template used to render the response. Actual template(s) used: /home/ben/Repos/myrepo/myproject/templates/staff/update_record.jinja
  </code></pre>
  
  <p>Is there something I need to add in <code>settings.TEMPLATES</code> or anywhere else to make this return the relative path rather than the absolute path?</p>