Use bundler

Ok, I've been using it for several projects, and it brings some much-needed sanity to the hell that is Ruby Gems!

```
# Gemfile
source 'https://rubygems.org'

gem 'capistrano', '~> 2.15'
gem 'capistrano-ext', '~> 1.2.1'
gem 'colored', '~> 1.2'
```
