license: handle "Public Domain".

[MikeMcQuaid]

Allow :public_domain to be used as a licenscommite value and ensure that it's handled correctly by brew info, brew audit and brew install.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• 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? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

[MikeMcQuaid]

Thanks for review @jonchang!