Change XXX.minorUnits to 0 (#33)

Motivation:

ISO 4217 leaves the 'minorUnits' or 'exponent' of several currencies as 'undefined'. For example, both XTS and XXX have no specified value.

XTS was given the value of '1' arbitrarily for the purposes of testing rounding behaviors, while XXX should be 0 as "no currency" is used
in transactions with XXX.

Modification:

- Change: `XXX.minorUnits` to be `0`

Result:

`XXX` currency values should now be whole number representations.

Resources/ISO4217.csv

@@ -158,4 +158,4 @@ LESOTHO,Rand,ZAR,710,2,,
 ZAMBIA,Zambian Kwacha,ZMW,967,2,,
 ZIMBABWE,Zimbabwe Dollar,ZWL,932,2,,
 ,Code reserved for testing,XTS,963,1,,
-,No currency,XXX,999,2,,
+,No currency,XXX,999,0,,

Sources/Currency/ISOCurrencies.swift

@@ -2252,7 +2252,7 @@ public struct XXX: CurrencyProtocol, CurrencyMetadata {
   public static var name: String { return "No currency" }
   public static var alphabeticCode: String { return "XXX" }
   public static var numericCode: UInt16 { return 999 }
-  public static var minorUnits: UInt8 { return 2 }
+  public static var minorUnits: UInt8 { return 0 }
 
   public var minorUnits: Int64 { return self._minorUnits }