Is it really necessary to depend on netstandard1.5 and not lower version?
FSharp.Core for F# 4.0 is based on .NET Portable Profile 259 (portable-net45+netcore45+wpa81+wp8) which is netstandard1.0 according to this

I'm just wonder if it is really necessary and Microsoft.FSharp.Core.netcore uses something special from netstandard1.5 API surface not included in netstandard1.0