Well, you can be 100% sure the final decision to commission a second season was made many months ago (pre-production, writing etc. for it would have been ongoing for months if they are to film this summer) and has nothing to do whatsoever with the ratings for the first episode. The announcement was made to coincide strictly as a matter of communication strategy. It just looks really good to announce you renew the show right after awesome ratings (and as HBO is as likely to lose some viewers for this show in the next weeks as gain new ones, it's logical to take no chance and do this now).