Recompiled for Windows 8.1

Hi Marice,

I could not get your SteamSmartie plugin to work on Windows 8.1, I kept getting errors regarding the Newtonsoft.Json DLL. I simply recompiled it to have the target framework as .NET 4.5 and I also updated NewtonSoft.Json to 6.0.5 from 6.0.4 while I was at it. Works fine on my machine now. I am creating this pull request in case you're interested in merging the (very minor) changes.

Thanks for your work on this.

Cheers,
Jan