Unicorn not started when booting

When (re)booting the Vagrant box, Unicorn isn't started automatically.

To fix this manually, we either have to deploy again with Capistrano or simply run:

```sh
practicing-ruby-web (master)]$ cap vagrant unicorn:start
    triggering load callbacks
  * 2013-10-30 12:14:03 executing `vagrant'
    triggering start callbacks for `unicorn:start'
  * 2013-10-30 12:14:03 executing `multistage:ensure'
  * 2013-10-30 12:14:03 executing `unicorn:start'
    triggering before callbacks for `unicorn:start'
  * executing "if [ -e \"/home/deploy/current/config/unicorn.rb\" ]; then\\\n          UNICORN_CONFIG_PATH=/home/deploy/current/config/unicorn.rb;\\\n        else\\\n          if [ -e \"/home/deploy/current/config/unicorn/production.rb\" ]; then\\\n            UNICORN_CONFIG_PATH=/home/deploy/current/config/unicorn/production.rb;\\\n          else\\\n            echo \"Config file for \"production\" environment was not found at either \"/home/deploy/current/config/unicorn.rb\" or \"/home/deploy/current/config/unicorn/production.rb\"\";\\\n            exit 1;\\\n          fi;\\\n        fi;\\\n\\\n        if [ -e \"/home/deploy/shared/pids/unicorn.pid\" ]; then\\\n          if  kill -0 `cat /home/deploy/shared/pids/unicorn.pid` > /dev/null 2>&1; then\\\n            echo \"Unicorn is already running!\";\\\n            exit 0;\\\n          fi;\\\n\\\n           rm /home/deploy/shared/pids/unicorn.pid;\\\n        fi;\\\n\\\n        echo \"Starting Unicorn...\";\\\n        cd /home/deploy/current &&  RAILS_ENV=production BUNDLE_GEMFILE=/home/deploy/current/Gemfile bundle exec unicorn -c $UNICORN_CONFIG_PATH -E deployment -D ;"
    servers: ["practicingruby.local"]
    [practicingruby.local] executing command
 ** [out :: practicingruby.local] Starting Unicorn...
    command finished in 2409ms
```

@sandal Maybe it's a good idea to have a startup script that takes care of starting Unicorn for us, similar to what I've done for God (#6).