Nuget upgrade neccessary to build solution file

I upgraded NuGet to product version 2.8.1 as it's a required to have at least 2.8 in order to build the application with the _CreateNewNuGetPackage Powershell commands. Haven't tried contributing to github before, so if I need to change something, let me know. Left it on master as it's not likely to affect any other possible branches.