Add ImageMagick to default Rails install

This may not be the best place for it, but I figured I would bring it up for discussion. I am using this repository to stage a somewhat simple Rails application and came across a need for ImageMagick as part of the popular [Paperclip](https://github.com/thoughtbot/paperclip) gem. As I went to foil-ball it on the server, it occurred to me that wouldn't be a very good idea in terms of adhering to good configuration management.

This PR adds a reference to the upstream `imagemagick` cookbook from OpsCode (if I did it right) to be installed as part of the Rails stack.