Add availability check for iOS 8 attributes.

Source/ConstraintAttributes.swift

@@ -125,32 +125,34 @@ internal struct ConstraintAttributes: OptionSetType, BooleanType {
             attrs.append(.Baseline)
         }
         #if os(iOS)
-        if (self.contains(ConstraintAttributes.FirstBaseline)) {
-            attrs.append(.FirstBaseline)
-        }
-        if (self.contains(ConstraintAttributes.LeftMargin)) {
-            attrs.append(.LeftMargin)
-        }
-        if (self.contains(ConstraintAttributes.RightMargin)) {
-            attrs.append(.RightMargin)
-        }
-        if (self.contains(ConstraintAttributes.TopMargin)) {
-            attrs.append(.TopMargin)
-        }
-        if (self.contains(ConstraintAttributes.BottomMargin)) {
-            attrs.append(.BottomMargin)
-        }
-        if (self.contains(ConstraintAttributes.LeadingMargin)) {
-            attrs.append(.LeadingMargin)
-        }
-        if (self.contains(ConstraintAttributes.TrailingMargin)) {
-            attrs.append(.TrailingMargin)
-        }
-        if (self.contains(ConstraintAttributes.CenterXWithinMargins)) {
-            attrs.append(.CenterXWithinMargins)
-        }
-        if (self.contains(ConstraintAttributes.CenterYWithinMargins)) {
-            attrs.append(.CenterYWithinMargins)
+        if #available(iOS 8.0, *) {
+            if (self.contains(ConstraintAttributes.FirstBaseline)) {
+                attrs.append(.FirstBaseline)
+            }
+            if (self.contains(ConstraintAttributes.LeftMargin)) {
+                attrs.append(.LeftMargin)
+            }
+            if (self.contains(ConstraintAttributes.RightMargin)) {
+                attrs.append(.RightMargin)
+            }
+            if (self.contains(ConstraintAttributes.TopMargin)) {
+                attrs.append(.TopMargin)
+            }
+            if (self.contains(ConstraintAttributes.BottomMargin)) {
+                attrs.append(.BottomMargin)
+            }
+            if (self.contains(ConstraintAttributes.LeadingMargin)) {
+                attrs.append(.LeadingMargin)
+            }
+            if (self.contains(ConstraintAttributes.TrailingMargin)) {
+                attrs.append(.TrailingMargin)
+            }
+            if (self.contains(ConstraintAttributes.CenterXWithinMargins)) {
+                attrs.append(.CenterXWithinMargins)
+            }
+            if (self.contains(ConstraintAttributes.CenterYWithinMargins)) {
+                attrs.append(.CenterYWithinMargins)
+            }
         }
         #endif
         return attrs