Q:

Why have the Williams F1 team changed their livery and branding recently? 

I have noticed that the Williams F1 Racing Team have shed the Martini branding this last race (and maybe the race before as well; I don't recall).  It's nowhere to be found on the car, the racing overalls, even the name of the team has changed (although the official championship entry is registed to "Williams Martini Racing"). 
Most obvious is the omission of the red lines on the car's typical Martini livery. 
Has this to do with the specific venue, Abu Dhabi, where maybe alcoholic sponsoring is forbidden ? 

A:

Yes, there are some venues where alcohol advertising is banned, and any team sponsored by an alcoholic beverage company changes its livery for those races.   When Williams was sponsored by Anheuser-Busch, the team changed the branding on their car to Sea World Adventure Parks (owned by Anheuser-Busch) for non-alcohol venues. 
Alcohol advertising is currently banned in France and in many middle-east countries.   This is a growing movement, and there is international pressure on Formula 1 to ban alcohol sponsorship globally.