Update deprecated matcher protocol for RSpec 3.

The methods failure_message_for_should and failure_message_for_should_not
have been updated to failure_message and failure_message_negated respectively.
Alias to the old methods to remain backwards compatibility with RSpec 2.
Update NEWS
Bump to version 2.5.0
Require minitest instead of minitest/unit

to make Rail 4.1.0.beta1 happy