Add back limited support for Rails 2.3

cache_methods are included into ActiveRecord::Base, but benchmarking isn't included
since ActiveSupport::Notifications are only available on Rails >= 3.
fix get_caches_as_list by ensuring cache_ids are strings

`get_caches` returns a hash keyed by `#cache_id`, which on older versions of cache_fu
would return an integer. `#cache_id` now always returns a string so we have to convert
the arguments passed into `get_caches_as_list` to match.