[webkit-efl] Bump Ubuntu version to 15.10 to 16.04

Hello, Although I don't know how many people are interested in this mail, I'd like to notify to bump our base Ubuntu version from 15.10 to 16.04. I couldn't bump base Ubuntu version lately because there was layout test problem on 16.04. However I succeed to run layout test on 16.04, and looks like now is time to bump our base Ubuntu version. If there is any objection to bump, please let me know. If not, I'm going to bump Ubuntu on EFL bots. [EFL] Layout Test doesn't run on Ubuntu 16.04 https://bugs.webkit.org/show_bug.cgi?id=158141 gyuyoung.