replace depracated ".stub!" with "stub"

and get rid of the 1000 annoying warnings :)

Here is one of get as example:

DEPRECATION: stub! is deprecated. Use stub instead. Called from /Users/lucaspinto/Projects/rails_best_practices/spec/rails_best_practices/reviews/use_multipart_alternative_as_content_type_of_email_review_spec.rb:25:in `mock_email_files'.