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."

Conflicts:
	build/NuSpecs/tools/install.core.ps1
Makes sure that the very first build on a fresh machine works immediately (previously you'd get an error saying that some NuGet packages were restored and you need to run the build again).
Adds version comment to UmbracoVersion.txt and makes it possible to overrule by adding arguments to build.bat
Merge branch '7.1.9' into 7.2.0

Conflicts:
	build/Build.bat
	build/UmbracoVersion.txt
Only set comment to blank if there's command line arguments
Merge branch '7.1.9' into 7.2.0