Update ATF API URL

[tadhg-ohiggins]

The related documents API is being updated, and its URL will change to https://www.atf.gov/api/v1.1/ereg/

We should change our code accordingly.

• [x] Test the staging URL (which we don't have yet) on our demo server.
• [x] Test the production URL on our demo server.
• [x] Push the production URL code to our production server.

[yowill]

@tadhg-ohiggins Is this finished/clear to close?

[tadhg-ohiggins]

Yup, closing.