Proper default setting for ALLOWED_HOSTS.

From the Django documentation:

The default value of this setting in Django 1.4.4+ is ['*'] (accept any
host) in order to avoid breaking backwards-compatibility in a security
update, but in Django 1.5+ the default is [] and explicitly configuring
this setting is required.

https://docs.djangoproject.com/en/1.4/ref/settings/#allowed-hosts