Improves nuget web.config transform

D'OH! Well this was the whole premise for the PR. So.. in Umbraco we have `configsource=etc` already, when this NuGet installs it just updates the existing clientDependency node and adds the version to it... Which is not allowed. Now I also get why you don't have an example config in the standalone package. :-)

So.. never mind, I'll fix it in the Umbraco transform instead!