Fix upercase System.XML reference to enable mono builds
Fix upercase System.XML reference to enable mono builds

For some reason, the JsonFx.csproj uses a reference to System.XML
instead of System.Xml. The uppercase .XML is not understood by mono's
xbuild implementation and will result in a build failure. This fix
changes that and now build succeeds using latest mono and xbuild tool.

jsonfx/jsonfx