fixes fsharp/FAKE#165 by using a FSharpTargetsPath property
Visual Studio 2013 and MSBuild 12 use F# 3.1
Visual Studio 2012 and MSBuild 4 use F# 3.0
fallback is open source F# in ..\..\..\tools\FSharp
Merge pull request #204 from ctaggart/issue165try3

fixes fsharp/FAKE#166 by using a FSharpTargetsPath property