Attribute Unavailable warning on iOS versions prior to 8.0
Since I had already created the constraints, the answer was to click on the constraint, and then look in the Utilities bar (on the right). As shown in the picture below, click on an item and then make sure that "relative to margin" is unchecked.
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.
If you can't find where the warning is coming from you can use this grep to find it.
Run this in Terminal in your project directory:
grep -C 6 -nr Margin .The 6 says how many lines of context to show before and after the match.