http://webapps.stackexchange.com/questions/30654/in-google-drive-how-can-you-link-directly-to-download-a-zip-file-and-not-view
For the moment, you have to build the URL manually.
https://docs.google.com/uc?export=download&id=YourIndividualID
Where
YourIndividualID is the ID of the respective document (zip file). You get it either from the URL or by clicking File → Share and copying it from the sharing URL.
For example, you may get something like this from google doc:
https://docs.google.com/file/d/YourIndividualID/edit?usp=sharing
