zeus: trying zeus as preloading tool for rails.

* Github: https://github.com/burke/zeus
* Trello: https://trello.com/c/PSObYXMz/605-zeus
* Railscasts: http://railscasts.com/episodes/412-fast-rails-commands
preloader: switching from zeus to spring.

* Trello: https://trello.com/c/GwsBlXLv/606-spring
* Github: https://github.com/rails/spring