Merge pull request #143 from devxoul/availability

Add availability check for iOS 8 and OS X 10.10 APIs.

Source/Constraint.swift

@@ -138,25 +138,25 @@ internal class ConcreteConstraint: Constraint {
     }
     
     internal override func activate() -> Void {
-        if NSLayoutConstraint.respondsToSelector("activateConstraints:") && self.installInfo != nil {
+        guard #available(iOS 8.0, OSX 10.10, *), self.installInfo != nil else {
+            self.install()
+            return
+        }
         let layoutConstraints = self.installInfo!.layoutConstraints.allObjects as! [LayoutConstraint]
         if layoutConstraints.count > 0 {
             NSLayoutConstraint.activateConstraints(layoutConstraints)
         }
-        } else {
-            self.install()
-        }
     }
     
     internal override func deactivate() -> Void {
-        if NSLayoutConstraint.respondsToSelector("deactivateConstraints:") && self.installInfo != nil {
+        guard #available(iOS 8.0, OSX 10.10, *), self.installInfo != nil else {
+            self.install()
+            return
+        }
         let layoutConstraints = self.installInfo!.layoutConstraints.allObjects as! [LayoutConstraint]
         if layoutConstraints.count > 0 {
             NSLayoutConstraint.deactivateConstraints(layoutConstraints)
         }
-        } else {
-            self.uninstall()
-        }
     }
     
     private let fromItem: ConstraintItem

Source/ConstraintAttributes.swift

@@ -125,6 +125,7 @@ internal struct ConstraintAttributes: OptionSetType, BooleanType {
             attrs.append(.Baseline)
         }
         #if os(iOS)
+        if #available(iOS 8.0, *) {
             if (self.contains(ConstraintAttributes.FirstBaseline)) {
                 attrs.append(.FirstBaseline)
             }
@@ -152,6 +153,7 @@ internal struct ConstraintAttributes: OptionSetType, BooleanType {
             if (self.contains(ConstraintAttributes.CenterYWithinMargins)) {
                 attrs.append(.CenterYWithinMargins)
             }
+        }
         #endif
         return attrs
     }