fix(Utilities): update VR namespace to be XR

In Unity 5.5.4 you get this error:
`Assets/VRTK/Source/Scripts/Utilities/VRTK_SharedMethods.cs(359,20):
 error CS0103: The name 'VRStats' does not exist in the current context`

The error disappears changing `VRStats` to `XRStats`.

Assets/VRTK/Source/Scripts/Utilities/VRTK_SharedMethods.cs

@@ -356,7 +356,7 @@ public static float GetGPUTimeLastFrame()
             float gpuTimeLastFrame;
             return (XRStats.TryGetGPUTimeLastFrame(out gpuTimeLastFrame) ? gpuTimeLastFrame : 0f);
 #else
-            return VRStats.gpuTimeLastFrame;
+            return XRStats.gpuTimeLastFrame;
 #endif
         }