Turns out that even on a simple library that uses another one, the dependent assembly doesn't get included. I think there is a bug tracking this but I couldn't quite find it.
You should be able to work around by explicitly listing the assemblies you care about in the nuspec. e.g.
<file src="bin\Debug\*.dll" target="lib\net40" />
This will include all dll's that ended up in the bin folder, but you can be more selective if you like.