The issue is the one I quoted above. I can use a custom nuget path, tested that successfully, but the bad part is that it would force everyone that shares this core library to change their tfs build definitions to use the beta nuget path. Maintenance annoyance that I'm not willing to inflict on them.
Point being, humanizer 2.1 is incompatible with the latest rtm bits for tfs build and there isn't a nice way to maintain nuget betas separately.
I assumed you needed 3.5-beta2 since that's what comes with VS 2015.3, but I see that's not the case. That's good. Maybe I can get a server admin at my company to update "C:\Program Files\Microsoft Team Foundation Server 14.0\Tools\NuGet.exe" since it isn't prelease we're talking about anymore.