Rails 4.1

I was getting deprecation warnings about the `:terminator` string, so I replaced it with a lambda.
( [Rails code](https://github.com/rails/rails/blob/febfc57e56558bd6924b39a38f08d833a78e975b/activesupport/lib/active_support/callbacks.rb#L670-L728) )

I've also added [Appraisal](https://github.com/thoughtbot/appraisal) to make sure I didn't break anything in other rails versions.  To run the tests against each version use `appraisal rake test`.