This reverts commit 50c964df.

This was causing the flaky tests that #1508 started skipping.

This is the second time that `Formula.installed`'s cache has bitten
me with intermittent test failures, and I'd like it to be the last,
so I've made it so the cache is cleared automatically when a tab is
created. This _should_ mean that the cache is cleared any time it
needs to be, with the exception of when a Keg is created artificially
with no tab. I don't think there's anything I can do to automatically
handle that use-case, though.

Reorder and rename test files.

audit: only check previous formula version.

audit: check that there are any stable revisions.

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.

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.

Fixes the exception if `revision_map[formula.stable.version]` is `nil`.

hbc/cmd/style: bump rubocop-cask to 0.10.6

uninstall: remove nil requireds/dependents

development_tools: don't create Versions from `nil`

uninstall: skip some flaky tests for now.

This cause issues when e.g. using `debrew.rb` on a failing `system`
command in a formula.

These shouldn’t get here in the first place so this is a bit of a hack
pending a better fix.

These will be fixed properly later but this will avoid causing
unnecessary CI failures.

audit: fix "version should not decrease" (again).

- output the max version and current version to make the message more
  obvious.
- Don't compare the development spec version to the formula version and
  then complain the formula version is lower (it should always be!)

Warn developers when uninstalling a dependency

Add "null version" class, and return compiler versions/build versions as Version objects

Update RuboCop to 0.45.0.

Fix detection of self-extracting `.exe` files.

Works around Rubycop not liking method names that start with `is_`
by changing convention from singular to plural.

I think it's better that way anyway.

Suggested in #1084.

Made the existing warning output entirely to STDERR, because
previously the first line went to STDERR and subsequent ones went
to STDOUT.

Don't warn about unused `brew install` arguments.

This is a step closer to better argument handling but for now just
fixes the issue in #1217 where it starts complaining about options like
`--build-from-source` being used.

formula_versions: fix erroneous :stable in version_attributes_map