Fixed an error in page crawling which is likely caused by an API change in zombie.js

The .visit() method, the way it is being used here, expecting 3 variables in the callback (err, browser, status), should be a static method, rather than an instance method.

The instance method in zombie.js does not give the callback those 3 variables, causing "var links = browser.queryAll('a');" to throw an exception, as browser will be null.

Reference: https://github.com/assaf/zombie