Add default locale for currency

Babel tried to guess it from the LANG environment variable. But that's
not set in all environments, and would lead to an error in those cases.

Fixes #837

oscar/defaults.py

@@ -11,6 +11,7 @@ OSCAR_MAX_BASKET_QUANTITY_THRESHOLD = 10000
 
 # Currency
 OSCAR_DEFAULT_CURRENCY = 'GBP'
+OSCAR_CURRENCY_LOCALE = 'en_GB'
 
 # Max number of products to keep on the user's history
 OSCAR_RECENTLY_VIEWED_PRODUCTS = 20