Fix Swift 4 string compiler deprecation warning.

Zip/Zip.swift

@@ -148,7 +148,7 @@ public class Zip {
 
             var pathString = String(cString: fileName)
             
-            guard pathString.characters.count > 0 else {
+            guard pathString.count > 0 else {
                 throw ZipError.unzipFail
             }