Update lang/fsharp to 3.1.2.5.

[rpaulo]

Summary

Update fsharp 3.1.2.5. This unbreaks the port with Mono 4.0.

Test Plan

https://people.freebsd.org/~rpaulo/poudriere/build.html

% fsharpi

F# Interactive for F# 3.1 (Open Source Edition)
Freely distributed under the Apache 2.0 Open Source License

For help type #help;;

> let f = 1;;

val f : int = 1

> printfn "%d" f;;
1
val it : unit = ()
> #help;;

  F# Interactive directives:

    #r "file.dll";;        Reference (dynamically load) the given DLL
    #I "path";;            Add the given search path for referenced DLLs
    #load "file.fs" ...;;  Load the given file(s) as if compiled and referenced
    #time ["on"|"off"];;   Toggle timing on/off
    #help;;                Display help
    #quit;;                Exit

  F# Interactive command line options:

      See 'fsharpi --help' for options


>