Change update server address

[ghost]

All Submissions:

• Have you followed the Classic Commerce Contributing guideline?
• Does your code follow the repo standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Change the update server address to officialplugins.classicpress.net.

Closes #300

How to test the changes in this Pull Request:

1. Load up new class-wc-update-client.php file to a test site
2. Lower the version number in the main file
3. Visit backend and proceed with update
4. It works!

Screenshot - before:

Screenshot - after:

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?
• Have you included screenshots before/after your changes, if applicable?

[timbocode]

Yup.