Correct validators spec
Correct validators spec

This is part of the effort started at
https://github.com/thoughtbot/paperclip/issues/2199 to ensure the test
suite is green locally.

The validators spec was failing since the setup required was missing a
call to 'rebuild_class' and leaving the Dummy class in a bad state.

This PR adds the call to the before block setup.

victorngkp/paperclip,rpbaptist/paperclip,rpbaptist/paperclip,victorngkp/paperclip,gfvcastro/paperclip,gfvcastro/paperclip