Updates to use .NET 4.5

Some tooling (such as the .NET 4.5-only build of Mono) can only assume the presence of .NET 4.5 on the target box and not older .NET 2.0 MSBuild SDK components.  This updates FsLexYacc to make the FsLex.exe, FsYacc.exe and build tasks all .NET 4.5 DLLs.