According to this it's just a install and go for Visual Studio 2013:
In fact, installing the C# 6.0 compiler from this release involves little more than installing a Visual Studio 2013 extension, which in turn updates the MSBuild target files.
So just get the files from https://github.com/dotnet/roslyn and you are ready to go.
You do have to know it is an outdated version of the specs implemented there, since they no longer update the package for Visual Studio 2013:
You can also try April's End User Preview, which installs on top of Visual Studio 2013. (note: this VS 2013 preview is quite out of date, and is no longer updated)
So if you do want to use the latest version, you have to download the Visual Studio 2015.