Fix XmlProvider cross-compilation to F# 3.1

[ovatsus]

Currently compiling code that uses XmlProvider with VS2013 fails

[ovatsus]

After some investigation, I came to the conclusion F# 3.1 is not happy with this:

class XmlProvider+SetChoice : string option
    member Boolean: bool option with get
    TextRuntime.ConvertBoolean("", this)

    member Number: int option with get
    TextRuntime.ConvertInteger("", this)

    member String: string option with get
    TextRuntime.ConvertString(this)

and gives the error The type provider 'XmlProvider' used an invalid parameter in the ParameterExpression: this
F# 3.0 handled this fine

[ovatsus]

Ah turns out we weren't mapping option from FSharp.Core 4.3.1.0 to 4.3.0.0