Update rspec matcher protocol for rspec3.0

When I run rspec with `Paperclip::Shoulda::Matchers`, 

I could see the error like below. 
```
Paperclip::Shoulda::Matchers::HaveAttachedFileMatcher 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.
```

In RSpec3.0, failure messages methods in matchers are updated  to ` failure_message` and `failure_message_negated`

So I changed methods in matchers.