With Capistrano 3 out and Mavericks and Yosemite released... or if you had a recent clean install on of Rails and Capistrano on your development machine you may end up getting errors when trying to deploy a Rails 3.X app due to your machine having Capistrano 3.0 being installed.
If you are getting errors about Capistrano 3 being unable to read your deploy.rb file...
In your Gemfile, change your reference to Capistrano to the last version published in 2.0 (as of this writing).
group :development do gem 'capistrano', '~> 2.15.9' #other development gems... end
On your local machine, make sure you have that version installed
gem install capistrano -v 2.15.9