Removed call to with_metaclass as it is no longer needed

Subfieldbase is deprecated and scheduled to be removed with Django 1.10

Fixes #257

push_notifications/fields.py

@@ -58,7 +58,7 @@ def prepare_value(self, value):
 		return super(forms.CharField, self).prepare_value(value)
 
 
-class HexIntegerField(six.with_metaclass(models.SubfieldBase, models.BigIntegerField)):
+class HexIntegerField(models.BigIntegerField):
 	"""
 	This field stores a hexadecimal *string* of up to 64 bits as an unsigned integer
 	on *all* backends including postgres.