fix warning about renamed method in django 1.8

cmsplugin_filer_utils/__init__.py

@@ -7,8 +7,14 @@ def __init__(self, select_related=None):
         self._select_related = select_related
         super(FilerPluginManager, self).__init__()
 
-    def get_query_set(self):
-        qs = super(FilerPluginManager, self).get_query_set()
+    def get_queryset(self):
+        # Remove once support for Django 1.4/1.5 are dropped
+        try:
+            qs = super(FilerPluginManager, self).get_queryset()
+        except AttributeError:
+            qs = super(FilerPluginManager, self).get_query_set()
         if self._select_related:
             qs = qs.prefetch_related(*self._select_related)
         return qs
+
+    get_query_set = get_queryset