munkitools2: default to stable release

Posting for review:

Got around to fixing the munkitools2 recipe to work the same way as munkitools1. The default DOWNLOAD_URL is a base URL that gets scraped, and any override will just pass it through to 'url'.

Since this is GitHub I could be better served using the releases API, but at least for now, this was a quick attempt to just get parity with the 1.x recipe, and eventually it could be upgraded to use the API proper.

One thing would be to just make existing users of the recipe aware of the change of the default DOWNLOAD_URL.