I would advise against restricting the SecurityProtocol to TLS 1.1.
The recommended solution is to use
System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls
Another option is add the following Registry key:
Key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319
Value: SchUseStrongCrypto
It is worth noting that .NET 4.6 will use the correct protocol by default and does not require either solution.