Capistrano 2.0 compatibility

Since Capistrano 2.0 recipes must be wrapped in a configuration instance and tasks need to be namespaced. I don't have a long history of Capistrano use, but my short research on the topic led me to believe this is the case.

Also I needed to modify the rake task invocation to cd to deployment path and add RAILS_ENV environment variable to get everything working smoothly.

Tested with current Capistrano release (2.14.1).

Cheers!