Since other answers didn't work for me (Swift, Xcode 6.1.1) here I post my solution:
var url = NSURL(string: "itms://itunes.apple.com/de/app/x-gift/id839686104?mt=8&uo=4")
if UIApplication.sharedApplication().canOpenURL(url!) {
UIApplication.sharedApplication().openURL(url!)
}