Unfortunately, you can't. Google search now exclusively uses HTTPS, and all search result clicks go through an intermediate URL that removes the keywords. The URL you posted above appears to be that intermediate URL, and that's all you get as the referrer.
The reasoning behind this is that each user of Google often gets personalised results based on their search history, so if a visitor's referrer containing the search terms for your site it could potentially 'leak' information about that user.
You should create a Google Webmaster Tools account, which will allow you to see anonymised summaries of what keywords sent people to your site.