Fix currency default value to play nice with django1.7 migrations

### What is the problem / feature ?

When running oscar on Django 1.7 in an application where you have modified `OSCAR_DEFAULT_CURRENCY` the new migration engine detects the change as something it needs to generate a migration for.

### How did it get fixed / implemented ?

Changed the default value to be a function instead.  This prevents the migration engine from interpreting a settings change as something it needs to generate a migration for.

### How can someone test / see it ?

Change the `OSCAR_DEFAULT_CURRENCY` setting, run `makemigrations` and see that no new migrations are generated.

*Here is a cute animal picture for your troubles...*

![tumblr_kxwxn9h8od1qa6c5so1_500](https://cloud.githubusercontent.com/assets/824194/4549944/542942b8-4e62-11e4-8b16-bf6ef2248e21.jpg)