update to support swift4+

Author: gaoxiang

Source/iOS/UIImage+Extensions.swift

@@ -22,8 +22,14 @@ extension UIImage {
 
   /// Convert to data
   func cache_toData() -> Data? {
+    #if swift(>=4.2)
     return hasAlpha
-      ? self.pngData()
-      : self.jpegData(compressionQuality: 1.0)
+      ? pngData()
+      : jpegData(compressionQuality: 1.0)
+    #else
+    return hasAlpha
+      ? UIImagePNGRepresentation(self)
+      : UIImageJPEGRepresentation(self, 1.0)
+    #endif
   }
 }