Fix deprecation warning re: 'flatMap'

Swift 4.1 warns:

> 'flatMap' is deprecated: Please use compactMap(_:) for the case where closure returns an optional value

Lib/KeychainAccess/Keychain.swift

@@ -778,7 +778,7 @@ public final class Keychain {
     }
 
     public func allKeys() -> [String] {
-        return type(of: self).prettify(itemClass: itemClass, items: items()).flatMap { $0["key"] as? String }
+        return type(of: self).prettify(itemClass: itemClass, items: items()).compactMap { $0["key"] as? String }
     }
 
     public class func allItems(_ itemClass: ItemClass) -> [[String: Any]] {