change_list.html: fix compress command of django-compressor

[PetrDlouhy]

The django-compress raises following errors during run of compress management command and also compilescss command of django-sass-processor:

# python manage.py compress
Compressing...
Error parsing template adminsortable2/change_list.html: Invalid template name in 'extends' tag: ''. Got this from the 'base_change_list_template' variable.