The issue here is that you just Added the reference to System.IO.Compression it is missing the reference to System.IO.Compression.Filesystem.dll
And you need to do it on .net 4.5 or later (because it doesn't exist on older versions).
I just posted a script on TechNet Maybe somebody would find it useful it requires .net 4.5 or 4.7
https://gallery.technet.microsoft.com/scriptcenter/Create-a-Zip-file-from-a-b23a7530