Fixed the breadcrumb of the object_history template.

NOTE: this could conflict with projects that use django-reversion,
if the `VersionAdmin` is the last in the inheritance chain.

polymorphic/templates/admin/polymorphic/object_history.html

@@ -0,0 +1,6 @@
+{% extends "admin/object_history.html" %}
+{% load polymorphic_admin_tags %}
+
+{% block breadcrumbs %}
+  {% breadcrumb_scope base_opts %}{{ block.super }}{% endbreadcrumb_scope %}
+{% endblock %}