Follow rspec convention when including matchers

[lowjoel]

In master, matchers were merged in #52.

Instructions were to require "rspec/acts_as_matchers". My current rails_helper file looks like:

require 'rspec/rails'
require 'shoulda/matchers'
require 'cancan/matchers'

Maybe we should call it 'active_record/acts_as/matchers' instead