How To Use Nuget Packages in Cake Build Tool

build.cake:

var target = Argument("target", "Default");

Task("Default")
  .Does(() => {
     var client = new RestSharp.RestClient("www.test.com");
    Information("Hello World!");
});

RunTarget(target);

packages.config:

<?xml version="1.0" encoding="utf-8"?>
<packages>
    <package id="Cake" version="0.21.1" />
    <package id="RestSharp" version="105.2.3" />
</packages>

Error Received:

error CS0246: The type or namespace name 'RestSharp' could not be fou nd (are you missing a using directive or an assembly reference?)