Replace deprecated rspec methods with new ones

Hi!

Trying to use rspec-dns, I was faced with the following warning message. 


```
Deprecation Warnings:

`failure_message_for_should_not` is deprecated. Use `failure_message_when_negated` instead. ...

`failure_message_for_should` is deprecated. Use `failure_message` instead. ...

```

I've replaced the deprecated rspec methods with new ones.
(see: [Module: RSpec::Matchers::DSL::Macros::Deprecated](http://www.rdoc.info/github/rspec/rspec-expectations/RSpec/Matchers/DSL/Macros/Deprecated))