README.md is wrong about initial setup

[payne911]

Currently using v1.2.48.

We are advised to start with RestCredentials creds = new BullhornRestCredentials();, but it doesn't work.

In fact, BullhornRestCredentials does not implement RestCredentials and thus this operation is illegal in Java.

The solution for the users of this library is quite trivial: BullhornRestCredentials creds = new BullhornRestCredentials();. But that's not really idea.

This should be a fairly easy fix in the codebase which would be backward compatible with the current state of things. I just thought I'd let you know so that you can update either the README or the code.