Beta Release Notes Don't Point to the Right Place

what's new / release notes dont work with beta versions.

Version

GitHub Desktop version: [1.0.10-beta0]

Steps to Reproduce

1. Open app.
2. See new update.
3. Click what's new link or release notes in About screen.

Expected behavior: See release notes for beta version.

Actual behavior: See release notes for last release version.

Reproduces how often: 100%

Additional Information

More than will to help solve this issue.

I know that #2774 might solve this differently.

I think https://desktop.github.com/release-notes/beta would be cool too.