update base url to stop using deprecated endpoint
update base url to stop using deprecated endpoint

Latest documentation has all endpoints pointing to api.hellofax.com. See
https://faq.hellosign.com/hc/en-us/articles/216809878-Complete-Walkthrough-for-Setting-Up-The-HelloFax-API

In addition, I've received notive from hellofax that they will deprecate the
old url starting on January 12, 2020.

maletor/hello_fax