I just installed the 'specflow.nunit' package via nuget, but the project containing the specs want compile anymore, because of the `SpecFlowNUnitExtension` class that is added to the project is missing references to nunit.core (more specifically the 'NUnit.Core' and 'NUnit.Core.Extensibility' namespaces can't be found). Anyone else had the same issue? Have you managed to solve it?