Added newton soft json in entry.cs

Author: cs-raj

Contentstack.Core/Models/Entry.cs

@@ -8,7 +8,8 @@
 using System.Threading.Tasks;
 using Contentstack.Core.Internals;
 using Contentstack.Core.Configuration;
-
+using Newtonsoft.Json;
+
 namespace Contentstack.Core.Models
 {
     /// <summary>