Attribute Unavailable warning on iOS versions prior to 8.0
When you add constraints in Xcode 6, you have the option "Constrain to margins" which create a different constraint and can't be read, for example, in Xcode 5.Create the constraints without this options and the warnings should go away. This constraints will be available on iOS 8.
If you have already created the constraints, we can change it clicking on the constraint and editing it. When selecting the second item, you can uncheck the "Relative to margin" option (as explained in the GLee answer.