Fix Unity build

Summary:
YogaValue.cs(20,30): error CS1644: Feature `expression bodied members' cannot be used because it is not part of the C# 4.0 language specification
    YogaValue.cs(21,28): error CS1644: Feature `expression bodied members' cannot be used because it is not part of the C# 4.0 language specification
Closes https://github.com/facebook/yoga/pull/316

Reviewed By: emilsjolander

Differential Revision: D4381190

Pulled By: splhack

fbshipit-source-id: fea157ac03944260d87b9f137bf966605c01a10e

csharp/Facebook.Yoga/YogaValue.cs

@@ -17,8 +17,21 @@ namespace Facebook.Yoga
         private float value;
         private YogaUnit unit;
 
-        public YogaUnit Unit => unit;
-        public float Value => value;
+        public YogaUnit Unit
+        {
+            get
+            {
+                return unit;
+            }
+        }
+
+        public float Value
+        {
+            get
+            {
+                return value;
+            }
+        }
 
         public static YogaValue Pixel(float value)
         {