Versions of the format 0.x.y should be considered stable

[ManuelHu]

Fixes #338, fixes #358

This will consider all versions with a maj́or version of 0 as stable when using the gnome checker.

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This is currently not covered by tests, as alleyoop only has one version branch 0.9.x: https://github.com/flathub/flatpak-external-data-checker/blob/890dd4b8d6fca69539ccdc5c53727f55716877fc/tests/org.gnome.baobab.json#L15-L18
this will not trigger the behaviour, as it will fallback to the single unstable version branch if no other stable version is available.

[gasinvein]

I've fixed the unrelated test failure (timeout on httpbin.org) and rebased your branch.

Although I'm not entirely sure this is correct. Doesn't Gnome use the odd-for-unstable version scheme for 0.x.y versions, too?

[hfiguiere]

I think #374 would help a lot.