Merge pull request #9 from drxlx/swift-4

Swift 4 migration.

Author: maxkramer

Example/Pods/Pods.xcodeproj/project.pbxproj

@@ -71,12 +71,12 @@ open class SwiftLuhn {
 
         let formattedCardNumber = cardNumber.formattedCardNumber()
 
-        guard formattedCardNumber.characters.count >= 9 else {
+        guard formattedCardNumber.count >= 9 else {
             throw CardError.invalid
         }
 
-        let originalCheckDigit = formattedCardNumber.characters.last!
-        let characters = formattedCardNumber.characters.dropLast().reversed()
+        let originalCheckDigit = formattedCardNumber.last!
+        let characters = formattedCardNumber.dropLast().reversed()
 
         var digitSum = 0