Fix a Django 1.7 issue with importing INSTALLED_APPS modules

I ran into this issue because I was trying to use the new [SimpleAdminConfig](http://django.readthedocs.org/en/1.7.x/ref/contrib/admin/index.html#django.contrib.admin.apps.SimpleAdminConfig) in Django 1.7. Now that INSTALLED_APPS can contain classes (and not just modules), a possible ImportError needs to be caught within the preview autodiscover function.

According to the 1.7 release notes, the proper way to access "INSTALLED_APPS" is through the new "App registry", but if we did that here then imagekit would become incompatible with previous versions of Django.