Check to see if INSTALLED_APPS is a tuple.

Otherwise this fails.

Should solve [this](https://groups.google.com/forum/?fromgroups=#!topic/django-non-relational/NA02zAqCktw) question on the [Django non-relational Google Group](https://groups.google.com/forum/?fromgroups=#!forum/django-non-relational)

  File "/usr/local/lib/python2.7/dist-packages/django_mongodb_engine-0.4.0-py2.7.egg/django_mongodb_engine/__init__.py", line 13, in <module> 
    settings.INSTALLED_APPS.insert(0, 'django_mongodb_engine') 
AttributeError: 'tuple' object has no attribute 'insert'