Fix: set default currency to USD and document why
Fix: set default currency to USD and document why

1. Why is this change necessary?
Because google doesn't support the MZN currency in e-commerce tracking.

2. How does it address the issue?
By setting the default e-commerce tracking currency to USD. And
documenting that the param should always be in the list of accepted
currencies.

3. What side effects does this change have?
None.

uxmz/tracking