Another attempt at fixing `brew audit` issues around detecting
`revision` and `version_scheme` changes correctly. First done in #1754
and #2086 (reverted in #2099 and #2100).
To ease future debugging a `ph` helper has been added to print a hash
and a series of RSpec tests to verify that the `revision`,
`version_scheme` and `version` formula version audits behave as
expected.
Fixes#1731.