What is the preferable way to consume asynchronously JSON API web service?

Hi,

I want to use JSON API for foreign exchange rates and currency conversion - http://fixer.io/

I've tried to use System.Net.Http HttpClient, but couldn't add it to my project because NuGet version isn't appropriate:
The 'System.Net.Http 4.0.0' package requires NuGet client version '3.0' or above, but the current NuGet version is '2.8.5.0'.

What alternatives do I have?

Regards,
Mike.