Update to support carrierwave 0.10.0

Carrierwave 0.10.0 changed some I18n paths for the error messages, making it now available on a per model/attribute level, so the tests had to be changed to use the new I18n path.

Also, the test for after removal of an image was explicitly requiring the image_identifier to be an empty string instead of just blank, allowing also the value nil, that is what is returned now in carrierwave 0.10.0.

Hope this also fixes the tests that were failing from PR #121 when tested against carrierwave-master.