How to upgrade msbuild to C# 6?

I want to use C# 6 in my project (null propagation, other features).

I've installed VS 2015 on my PC and it works brilliantly and builds test code like

var user = new SingleUserModel(); //all model fields are null
var test = user.User?.Avatar?["blah"];

But when I push my project to the repo and CI starts to build it, build fails because of unsupported ?.

I've installed VS2015 on CI server too but looke like it doesn't use it. What can I do?

CI - CruiseControl .NET Builds with C:\Windows\Microsoft.NET\Framework\v4.0.30319\MSBuild.exe