Don't include Cake.Core and Cake.Common in NuGet package

[pascalberger]

The NuGet package for version 2.0 contains Cake.Core, Cake.Common and JetBrains.Annotations.dll in lib\net461 folder. They should not be part of the NuGet package, as they are not required.

[nils-a]

fixed in 5610b50