Add dependency on System.Runtime.Serialization.Primitives to make up for JSON.NET not including it

Fixes aspnet/Mvc#3946

Author: pranavkm

src/Microsoft.AspNetCore.JsonPatch/project.json

@@ -22,6 +22,7 @@
         "System.Reflection.Extensions": "4.0.1-*",
         "System.Resources.ResourceManager": "4.0.1-*",
         "System.Runtime.Extensions": "4.1.0-*",
+        "System.Runtime.Serialization.Primitives": "4.1.0-*",
         "System.Text.Encoding.Extensions": "4.0.11-*"
       }
     }