It looks like the method stringByAppendingPathComponent is removed in Swift 2.0, so what the error message suggests is to use:
let writePath = NSURL(fileURLWithPath: NSTemporaryDirectory()).URLByAppendingPathComponent("instagram.igo")
Update:
URLByAppendingPathComponent() has been replaced by appendingPathComponent() so instead do:
let writePath = NSURL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent("instagram.igo")