Fix for tracking ad-related events in Mercury

@gabrys @hakubo This feels hacky but helps to let through the analytics events for ads, which are using tracker.js from the app stack.

There is one more solution proposed by @gabrys to override wikia.tracker with our own Mercury tracking module, which implements the same API as the app's one. This solution has some donwsides (we need to keep track of Mercury tracker in the app code, also we would have to override wikia.tracker with the whole utils module, which contains 'track' method in Mercury and tracking methods don't seem to be compliant so it would require further changes in our 'track' method).

What do you guys think is the best approach?
We need to slice arguments anyway, so there's a proper Array in the _gaq.push method, as it uses Array prototype methods inside.