The gem should have a change log

[gonzalo-bulnes]

As a developer who considers updating Dredd Hooks Ruby
In order to have a clear idea of which changes I should expect
I want them described in a CHANGELOG

As an Apiary developer
In order to make it consistent and easy to maintain
I want the Dredd Hooks Ruby to be similar to other Apiary change logs

See gonzalo-bulnes#2 and #5 (comment)

There is some way we do create Changelog for Dredd. It would be nice to have the process unified. Please note I don't think the current state is completely ideal - apiaryio/dredd#440, suggestions on improvements welcome.