Added #if swift(>=4.1) notation

Thanks to Andrew Bennet for suggestion!

SwiftyStoreKit/InAppReceipt.swift

@@ -96,7 +96,12 @@ internal class InAppReceipt {
         let filteredReceiptsInfo = filterReceiptsInfo(receipts: receipts, withProductIds: [productId])
         let nonCancelledReceiptsInfo = filteredReceiptsInfo.filter { receipt in receipt["cancellation_date"] == nil }
 
-        let receiptItems = nonCancelledReceiptsInfo.compactMap { ReceiptItem(receiptInfo: $0) }
+        #if swift(>=4.1)
+            let receiptItems = nonCancelledReceiptsInfo.compactMap { ReceiptItem(receiptInfo: $0) }
+        #else
+            let receiptItems = nonCancelledReceiptsInfo.flatMap { ReceiptItem(receiptInfo: $0) }
+        #endif
+        
         // Verify that at least one receipt has the right product id
         if let firstItem = receiptItems.first {
             return .purchased(item: firstItem)
@@ -133,7 +138,11 @@ internal class InAppReceipt {
 
         let receiptDate = getReceiptRequestDate(inReceipt: receipt) ?? date
 
-        let receiptItems = nonCancelledReceiptsInfo.compactMap { ReceiptItem(receiptInfo: $0) }
+        #if swift(>=4.1)
+            let receiptItems = nonCancelledReceiptsInfo.compactMap { ReceiptItem(receiptInfo: $0) }
+        #else
+            let receiptItems = nonCancelledReceiptsInfo.flatMap { ReceiptItem(receiptInfo: $0) }
+        #endif
 
         if nonCancelledReceiptsInfo.count > receiptItems.count {
             print("receipt has \(nonCancelledReceiptsInfo.count) items, but only \(receiptItems.count) were parsed")
@@ -163,12 +172,21 @@ internal class InAppReceipt {
                 return (expirationDate, $0)
             }
         } else {
-            return receiptItems.compactMap {
-                if let expirationDate = $0.subscriptionExpirationDate {
-                    return (expirationDate, $0)
+            #if swift(>=4.1)
+                return receiptItems.compactMap {
+                    if let expirationDate = $0.subscriptionExpirationDate {
+                        return (expirationDate, $0)
+                    }
+                    return nil
                 }
-                return nil
-            }
+            #else
+                return receiptItems.compactMap {
+                    if let expirationDate = $0.subscriptionExpirationDate {
+                        return (expirationDate, $0)
+                    }
+                    return nil
+                }
+            #endif
         }
     }