Update NuGet Dependency to OpenTK 2.0

[MutterOberin]

Update Dependency for Nuget Package from OpenTK.Next to OpenTK 2.0 Release

OpenTK Released 2016-09-23

[Patrick Yates-Jones]

Thanks for the heads up, I'll do this soon

[MutterOberin]

Thanks!

[Patrick Yates-Jones]

I've just published QuickFont 4.4 which updates the OpenTK dependency to 2.0. Feel free to open a new issue if anything is wrong with it :)