Only try show lightbox if this is actually a gallery

At the moment appending `?index=1` to any page other than a gallery will try fire off the lightbox only to error.

    http://www.theguardian.com/sport/cycling?index=2&view=responsive

![gallery](https://cloud.githubusercontent.com/assets/76709/4197063/517707bc-37e2-11e4-9b75-8786a5860011.png)