Fix loading of F# 3.1 .fsproj files

The F# project files generated by VS2013 have a project reference like this:

    <Reference Include="FSharp.Core, Version=$(TargetFSharpCoreVersion), Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a">

Mono Develop / Xamarin Studio currently crash while opening those .fsproj files

This pull requests fixes this problem by replacing all $(PropertyName) patterns in project references. It's not a complete solution for having property replacements in references, as it doesn't check for the Conditions, but it covers the F# projects case.