Fix compatibility with F# 3.1

Code compiled with VS2013 that used FSharp.Data wasn't working because FSharp.Data still targets F# 3.0. Now we detect that case and also replace the FSharp.Core reference like we do for portable class libraries and silverlight (works even if FSharp.Data.DesignTime.dll is compiled in VS2012)

WIP - XmlProvider still is not working, probably still misses a ToDesignTime somewhere, as in this scenario even in full framework we replace assemblies (silverlight didn't support xml, so the problems are probably there since ever). But at least it now fails at compile time instead of at runtime