This documentation is for QlikView 11. Not sure if this is still working for QlikView 12.
In the documentation on page 21 it describes you have to use the dll's in c:\Program Files\QlikView\Server\WebServer.
I think the correct path for QlikView 12 is: C:\Program Files\QlikView\Server\QlikViewClients\QlikViewAjax\bin
Make sure you reference these dll's in your solution.
I think the call to AddVersionHeader method is not necessary for the solution to work. But you will need the Context object. I believe you can obtain the context object using de QvIISWebServer.dll, this has a GetContext method.
Try to reference the mentioned dll's and remove that line of code.