[appveyor] Use simbody's project nuget feed.
[appveyor] Use simbody's project nuget feed.

[skip travis]
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