Add unicorn
Setup `config/unicorn.rb`

Use `dotenv` gem to read `.env` file and update it when the server
starts or restarts. Configure number of unicorn workers via `ENV`
variable.
Add unicorn deploy tasks
Setup `capistrano-env` for managing ENV variables
Use unicorn web server in production [ci skip]

* Setup deploy tasks for capistrano using `capistrano-unicorn` gem
* Manage `.env` file on production using `capistrano-env` (via
  ecbypi/capistrano-env on GitHub)