manage FSharp.Core version using Paket

The purpose of this is to switch to use the FSharp.Core published to NuGet.
https://github.com/fsharp/fsharp/issues/352

Unfortunately, Paket causes a bunch more noise in the diff.
@forki Looking at this again, I'm not sure `FAKE.Core` package should have a dependency on `FSharp.Core`. You may wish to revert this change. https://www.nuget.org/packages/FAKE.Core/