In the new \"Xcode 6\" the ability to change the center of measure of a button is missing.
In Xcode 5: http://imgur.com/jWHJp4v
Xcode 6: http://imgur.com/rsNayVZ
The feature has been removed from Xcode. I believe this is to account for the various sizes of devices now available, and as a result, the use of constraints is the way to go.