assert_block has been removed from minitest
Allows the custom message to be a callable object

* If a callable object is passed as the custom message, we call it in
  order to determine the message; otherwise we simply return it.

* The previous behavior set the message string when the model file is
  loaded. This new behavior allows the string to change depending on the
  state of the environment (for example if the I18n.locale has
  dynamically changed since the model was initially loaded).
Merge pull request #2 from howaboutwe/adds_callable_error_messages

Adds callable error messages