Fix Money deprecation warning w/ :default_currency

This removes the deprecation warning:

```
[WARNING] The default currency will change from `USD` to `nil` in the
next major release. Make sure to set it explicitly using
`Money.default_currency=` to avoid potential issues
```

config/initializers/money.rb

@@ -1 +1,2 @@
 Money.rounding_mode = BigDecimal::ROUND_HALF_EVEN
+Money.default_currency = Money::Currency.new('USD')