updates for Swift 4

[mathbeat]

This commit fixes the build errors associated with Swift 4.

[lucaventura]

@mathbeat Instead of returning a duplicate colors array to resolve the overlapping access error you can simply force unwrap colors since it is already force unwrapped in the if check:

 if colors!.count == 1 {
 -            colors!.append((colors?.first)!)
}

Can confirm this doesn't cause any crashes.