fixed availability checks

SwiftyStoreKit/PaymentQueueController.swift

@@ -123,7 +123,7 @@ class PaymentQueueController: NSObject, SKPaymentTransactionObserver {
         skPayment.applicationUsername = payment.applicationUsername
         skPayment.quantity = payment.quantity
         
-        if #available(iOS 12.2, tvOS 12.2, *) {
+        if #available(iOS 12.2, tvOS 12.2, OSX 10.14.4, *) {
             
             if let discount = payment.paymentDiscount?.discount as? SKPaymentDiscount {
                 skPayment.paymentDiscount = discount

SwiftyStoreKit/PaymentsController.swift

@@ -51,7 +51,7 @@ struct Payment: Hashable {
 public struct PaymentDiscount {
     let discount: AnyObject
     
-    @available(iOS 12.2, *)
+    @available(iOS 12.2, tvOS 12.2, OSX 10.14.4, *)
     public init(discount: SKPaymentDiscount) {
         self.discount = discount
     }