results in completion handler of performQuery should be typed

[cychiuae]

Currently the type of results in completion handler of performQuery is NSArray *. It makes us developers need to force cast the results to [SKYRecord] in Swift every time
i.e.

for todo in results as! [SKYRecord] {
    print ("Got a todo \(todo["title"])")
}

which is pretty ignoring.

We can easily fix that by change the type of results to NSArray<SKYRecord *> *.