在下载代码之前加入如下代码：

// 解决WebClient不能通过https下载内容问题
System.Net.ServicePointManager.ServerCertificateValidationCallback +=
	delegate(object sender, System.Security.Cryptography.X509Certificates.X509Certificate certificate,
	         System.Security.Cryptography.X509Certificates.X509Chain chain,
	         System.Net.Security.SslPolicyErrors sslPolicyErrors)
{
	return true; // **** Always accept
};

上述代码主要是为了跳过https的ssl验证。

参考文章：

https://stackoverflow.com/questions/12506575/how-to-ignore-the-certificate-check-when-ssl/18624335#18624335