[MRG] Downgrade base Docker image to 12.04

Reported by @pawelmhm: the default Splash Docker container (scrapinghub/splash) exits after processing http://www.menards.com/main/home.html URL. I can reproduce it. With old Dockerfile (based on Ubuntu 12.04) there are no segfaults / exits. I tried to to downgrade the image to 12.04 (while keeping other stuff - no supervisord, etc); it helped.