Make the nuget package compatible with NETCore

[ModernRonin]

Currently, trying to add the package to a Core library fails:

"error: Package TestStack.ConventionTests 3.0.1 is not compatible with netcoreapp1.0 (.NETCoreApp,Version=v1.0). Package TestStack.ConventionTests 3.0.1 supports: net40 (.NETFramework,Version=v4.0)
error: One or more packages are incompatible with .NETCoreApp,Version=v1.0."

Thanks,
Max