Set Unicorn environment according to RACK_ENV

[Tensho]

There are plenty of non-Rails applications (Grape, Sinatra, etc.) that could be served with Unicorn help. It seems to me relying on RACK_ENV as the more general sign may help in such a case. Also, I don't understand the purpose of "deployment" default option. I mean I understand, that it corrects default rack stack to use the only Rack::CommonLogger middleware, but I have never seen such setup in live. Usually people run staging or production envs within deployment.

[Tensho]

@ahorner, Please, could you give any idea, does it make sense?