use _meta.fields instead of get_all_field_names(), for Django-1.7 comp.

cash/admin.py

@@ -103,7 +103,7 @@ class SalesLogEntryAdmin(admin.ModelAdmin):
 	list_display = ('account', 'timestamp', 'product', 'count', 'unit_price')
 	list_filter = ['account', 'timestamp', 'product']
 	# Make sales log entries completely read-only
-	readonly_fields = SalesLogEntry._meta.get_all_field_names()
+	readonly_fields = map(lambda f: f.name, SalesLogEntry._meta.fields)
 
 class TransactionAdmin(admin.ModelAdmin):
 	list_display = ('account', 'timestamp', 'subject', 'description', 'amount')