Add alias for legacy RSpec matcher deprecation

When using Rspec 3.0.0 and writing tests for validating attachment size. I get the deprecation warning:

    Paperclip::Shoulda::Matchers::ValidateAttachmentSizeMatcher implements a legacy RSpec matcher
    protocol. For the current protocol you should expose the failure messages
    via the `failure_message` and `failure_message_when_negated` methods.