Update to rails 3.2.14

Rack >= 1.4.2 issues a security warning if there's no secret option
provided for Rack::Session::Cookie. This was a bug in rails. Although
rails users could safely ignore this warning, I decided to upgrade to a
more recent version of rails where the bug had been fixed.
Use RSpec and Capybara for testing
Introduce platforms for campaigns

* add feature spec for campaign management
* create platform model / active admin resource
* associate campaigns with platforms
* customize admin views to include platforms where appropriate