python_virtualenv_constants: upgrade virtualenv to 16.7.4

breadth-first search for dependents upgrade

Fixes Homebrew/brew#6386

Improve ZSH completion so local formula files are completable

* brew completion in ZSH currently doesn't complete local formula files for most commands.
* Updating the completion script to complete formula files as well as known formulas in places that it would makes sense to do so.
* This includesFixes to __brew_formula_or_ruby_files
  * Remove redundant call to uncached brew search
  * Add missing ""

Remove `brew cask automerge`.

messages_spec: handle frozen Homebrew.args.

This is an order dependent test failure fix as seen in
https://travis-ci.org/Homebrew/homebrew-test-bot/builds/574559080

Use `MacOSRequirement` for casks.

Add fallback for out-of-spec `Last-Modified` header.

Try caching the Docker image.

workflows/tests: hardcode coveralls token.

man: support multi-line paragraphs in comment-based docs

This is just used to set test coverage and GitHub Actions doesn't have
a good "secrets for forks" story yet.

manpages: fixes and rewording

GitHub Actions CI

Use active voice when applicable, better verbs & adjectives, preposition usage, proper tense, etc.

Reorder switches as force/quiet/verbose/debug, --installed before --all, and link to SPECIFYING FORMULAE from top of page.

Try to setup GitHub Actions CI.