Use `next` instead of `break` in `before_validation`

In Rails 4.1.4 it is excaption:

```
 Failure/Error: subject.valid?
 LocalJumpError:
   break from proc-closure
 # ./lib/paperclip/remote.rb:38:in `block in has_attached_file_with_remote'
 # ./spec/paperclip/remote_spec.rb:56:in `block (2 levels) in <top (required)>'
```