Avoid failing on deprecated currencies (such as ITL).

When attempting to render currencies that are no longer in use (such as `ITL`), I think it's acceptable to fall back on the supplied currency code and the default currency's settings rather than fail.

Old behavior:

    MoneyHelper.money_to_text(100, 'ITL')  # => Money::Currency::UnknownCurrency: Unknown currency `itl'

New behavior:

    MoneyHelper.money_to_text(100, 'ITL')  # => "ITL 100"