Ensure ConcreteConstraint cannot be subclassed

Source/Constraint.swift

@@ -63,7 +63,7 @@ public class Constraint {
 /**
     Used internally to implement a ConcreteConstraint
 */
-internal class ConcreteConstraint: Constraint {
+final internal class ConcreteConstraint: Constraint {
     
     internal override func updateOffset(amount: Float) -> Void {
         self.constant = amount