Have GithubApi.last_release ignore prereleases

[duckinator]

At the moment, if you (for example) release v6.0.0b1 then v6.0.0, you have to delete the v6.0.0b1 release first or v6.0.0 won't include all the relevant changes from the last stable version.

If GithubApi.last_release were to ignore prereleases (TODO: determine technical requirements for this), it would avoid this problem entirely. It might be worth renaming it to make it clear it doesn't include prereleases (maybe to last_stable_release?), but I'm not sure it matters as long as it's documented.

[duckinator]

The codebase filters out prereleases elsewhere -- namely bork.github.GithubRelease._get_release_info() -- so I think I'm going to consider this a bug, not a feature request.