Add dependency on FSharp.Core nuget package.
Add dependency on FSharp.Core nuget package.

Fixes #59.

sideeffffect/FsCheck,cloudRoutine/FsCheck,cloudRoutine/FsCheck,forki/FsCheck,fsharp/FsCheck,ErikSchierboom/FsCheck,fscheck/FsCheck,ploeh/FsCheck,forki/FsCheck,paulyoung/FsCheck,fscheck/FsCheck,paulyoung/FsCheck,jhamm/FsCheck