[appveyor] Workaround for MSVC Update 3 bug
[appveyor] Workaround for MSVC Update 3 bug

Update 3 to MSVC 2015 might be causing "internal compiler errors."

Thanks @carmichaelong for the detective work.

https://www.appveyor.com/updates
https://connect.microsoft.com/VisualStudio/Feedback/Details/2896447
https://connect.microsoft.com/VisualStudio/feedback/details/2869042/new-internal-compiler-error-only-in-vs2015-update-3
opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core