update package references to VS2017 syntax

[shiftkey]

There's a bunch of explicit project references from NuGet packages in the current source that can be replaced with PackageReference elements that use the NuGet package name.

An example:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net46</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0" />
    <PackageReference Include="xunit" Version="2.2.0" />
    <PackageReference Include="xunit.runner.visualstudio" Version="2.2.0" />
  </ItemGroup>
</Project>

Here's a guide to migrating the project files, which we'd probably need to do to enable this: http://www.natemcmaster.com/blog/2017/03/09/vs2015-to-vs2017-upgrade/

[hnrkndrssn]

I can take this one too 😁

[hnrkndrssn]

So it seems that we might not be able to update the Carnac project to the new VS2017 project format (or perhaps I'm misunderstanding something)...

If you are building WPF, Universal Windows, Xamarin, or ASP.NET 4 projects, you’ll have to stick with the old format

Issue for expanding support to the above project types dotnet/sdk#491

I can start by updating the Carnac.Logic and test project, and then we can upgrade the main project once WPF projects can support the new format.

[shiftkey]

@alfhenrik ugh, there's always a catch. Anyway, have a look and see what we can clean up.

[hnrkndrssn]

This is turning out to be a bit more effort than I first thought so I'm gonna sideline it for a bit while I do #125 as that has more value for our users 😁

[hnrkndrssn]

I'm gonna pick this one up again, we won't be able to update the main WPF project but we should be able to :fingerscrossed: update the other ones.