fixes #1074

Fixes issue #1074 - tolerates INSTALLED_APPS entries that aren't importable by catching ImportError and skipping. 

(Django-debug-toolbar, in its recommended 'quick setup' for Django >=1.7 <http://django-debug-toolbar.readthedocs.org/en/1.2/installation.html#quick-setup>, is not a module like Haystack expects.)