Remove references to absolute URI's

[Gman98ish]

Overview

https://github.com/ukfast/sdk-php/blob/master/src/Domains/Client.php#L22

In a few of the clients, instead of doing paths like v1/domains, we're giving an absolute path. We should be using the base path provided by guzzle by omitting the starting /registrars/. This means the base path can be changed if a different guzzle client is used