BuildableCurrencyUnit should be Serializable

I believe that `BuildableCurrencyUnit` is intended to be `Serializable`
because `Money` is `Serializable` and has a field named currency which
is a `CurrencyUnit`. So in order for `Money` to be `Serializable` all
fields have to be `Serializable` as well. The other option would be to
mark `CurrencyUnit` as `Serializable`.

 * make `BuildableCurrencyUnit` `Serializable`
 * add test that ensures `BuildableCurrencyUnit` is `Serializable`