Update .env.example according to code
Update .env.example according to code

refs #471

SkygearIO/skygear-server,SkygearIO/skygear-server,SkygearIO/skygear-server,rickmak/skygear-server,rickmak/skygear-server,SkygearIO/skygear-server,rickmak/skygear-server