audit: only check previous formula version.

[MikeMcQuaid]

• 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 tests with your changes locally?

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For calculating the stable/devel versions this should be sufficient as it's looking at origin/master so for a e.g. unmerged pull request this will stop complaining about mistakes outside the pull request itself.

CC @ilovezfs

This will silence all warnings for historic version mistakes (i.e.
before these audit checks were all enabled) which is normally a bad
thing but as this case would rely on modifying history to complete is a
good one.

[ilovezfs]

This is probably the right approach. However, it still leaves unanswered the question of whether going forward we will have to address every future mistake (ab)using version_scheme. If it only goes back one, then it'll probably be fine to ignore the audit failure if the time period during which it was wrong is short, since the next time the audit check should go back to passing.

[MikeMcQuaid]

since the next time the audit check should go back to passing.

Yes. This is basically just to address PRs from breaking these things silently.