Cannot uncheck targeting platform "Windows 8"

Hi,

in my Xamarin.Forms PCL Project (VS 2013 U3) i cannot uncheck Windows 8.
Every time i try it says "The Following will be automatically targeted because they support the same set of portable APIs: Windows 8".

I want to install the nuget package "ZXing.Net.Mobile Barcode Scanner" (1.4.6)" but i receive following error:

"Could not install package 'ZXing.Net.Mobile 1.4.6'. You are trying to install this package into a project that targets 'portable-net45+win+wpa81+MonoAndroid10+MonoTouch10', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author."

So what is the best solution to add the Barcode Scanner?

Thanks,
Johannes