Fix covariant cause build error in old compiler including Unity. issue #77.

Author: yfakariya

src/MsgPack/Serialization/INilImplicationHandlerOnUnpackedParameter.cs

@@ -2,7 +2,7 @@
 //
 // MessagePack for CLI
 //
-// Copyright (C) 2014 FUJIWARA, Yusuke
+// Copyright (C) 2014-2015 FUJIWARA, Yusuke
 //
 //    Licensed under the Apache License, Version 2.0 (the "License");
 //    you may not use this file except in compliance with the License.
@@ -18,14 +18,22 @@
 //
 #endregion -- License Terms --
 
+#if UNITY_STANDALONE || UNITY_WEBPLAYER || UNITY_WII || UNITY_IPHONE || UNITY_ANDROID || UNITY_PS3 || UNITY_XBOX360 || UNITY_FLASH || UNITY_BKACKBERRY || UNITY_WINRT
+#define UNITY
+#endif
+
 using System;
 
 namespace MsgPack.Serialization
 {
 	/// <summary>
 	///		Defines common interface for parameter of <see cref="NilImplicationHandler{TCondition,TPackingParameter,TAction,TUnpackedParameter}.OnUnpacked"/> method and its template methods.
 	/// </summary>
+#if NETFX_35 || UNITY
+	internal interface INilImplicationHandlerOnUnpackedParameter<TAction> : INilImplicationHandlerParameter
+#else
 	internal interface INilImplicationHandlerOnUnpackedParameter<out TAction> : INilImplicationHandlerParameter
+#endif
 	{
 		TAction Store { get; }
 	}