Rails4

in order to get Shopify/identity_cache running with Rails4, ar_transaction_changes needs to be updated to work with latest activerecord.

instead of monkeypatching lifecycle methods, it hooks into the callback-chain of activerecord.

i pinned the version of activerecord just to be explicit. i think that my changes are backwards compatible and should also work with rails3.

i also hooked it up to travis-ci: https://travis-ci.org/phoet/ar_transaction_changes/builds/7753157

feedback appreciated!