Should fix the build for real now. Sets GenerateSerializationAssemblies to Off which then doesn't produce umbraco.XmlSerializers.dll any more.

Should be safe as this is only used for updatechecker and packageinstaller. From:  http://hashtagfail.com/post/5255977780/sgen-loaderexceptions-error - "Visual Studio attempts to generate a serialization assembly containing the types in your solution, to improve XmlSerializer serialization performance, should you choose to serialize your types."