title: Constraint constant update not work on iOS 8
body: <p>I have a strange problem with my constraints on this simple UIViewController.
  I have only two elements in it and precisely:</p>
  
  <p>1 UIButton with these constraints:
  <img src="https://i.stack.imgur.com/eFW3L.png" alt="Button Constraints">
  1 PageControl with these constraints:
  <img src="https://i.stack.imgur.com/78VQf.png" alt="PageControl Constraints">
  The problem is in iOS 8. I have connected the BottomSpace Constraint of UIPageControl to change the constant value (that in iOS 7 works perfectly). In iOS 8 the value doesn't change without any error. My code is simple:</p>
  
  <pre><code>pageControlBottomConstraint.constant = 50
  </code></pre>
  
  <p>What is the difference between the two iOS version?</p>