- Note that Homebrew Cask issues should be filed elsewhere.
- Make `brew`/homebrew-core issue differential in simpler language
- Tell people to fix all `brew doctor` issues
- Refer to the issue checklist and make it even more clear and prominent
  that we may close it out immediately if they don't use it.
- Note that proposed new issues should be widely relevant.
- Don't tell people they need to `brew cask install java`; we handle
  this in most places with a Java requirement and probably want issues
  filed where we don't.
- Remove reference to Homebrew Dupes and replace with reference to
  Homebrew Cask to try and get people filing stuff there instead.

Fixes #2608

Travis CI tweaks.

Attempt to improve Travis CI. This time without any vendoring gems
nonsense.

fix date-based versioning

Revert gem vendoring experiment.

This reverts commit 3e4547f5, reversing
changes made to 6edf9382.

This reverts commit 23728729, reversing
changes made to 3e4547f5.

This reverts commit 8d948e4b, reversing
changes made to 23728729.

Travis CI tweaks

Tweak our Travis CI configuration and the one we suggest for taps to
be faster and use caching.

Tweak Gem vendoring.

If people have `HOMEBREW_RUBY_PATH` set then things explode in a rather
confusing fashion. Instead, run `bundle` for them with the arguments
that they'd want.

Also, move `macho` requires into the module itself; it's a pain having
to do everything for Bundler before requiring `pathname` which is a core
Ruby class.

Vendor all Homebrew's gems.

Homebrew's actually ended up using a fair few gems. While we want to
avoid Bundler at runtime (and this PR still does that, in fact uses
Bundler even less at runtime than it did before) writing our own version
to use at build-time seems redundant.

Remove unused `e` from `rescue Interrupt`.

Remove unused command_args

Extra command_args in gain_permissions_remove caused silent failure and path never gets deleted.

style: fix audit --online

This passed a symbol to `:except_cops` which caused a 💥.

style: don't require rubocop until installed.

This avoids an error when RuboCop installed installed yet.

CC @GauthamGoli FYI.

Remove `existing` from PATH when adding `Gem.bindir`.

audit: Allow skipping/selective running of cops and cops refactor

Install a bottle from an URL

caveats: tweak keg_only style

Install etc/var files on postinstall.

utils: create GEM_HOME when installing Gems.

Also, don't delete them after that. This means that `brew postinstall`
becomes a way to easily reinstall configuration files for any formula
without needing any changes to any bottles or requiring a reinstall.

It may not exist before Gem installation which means that the resulting
installed gem will not be found in the PATH.

Refactor PATH generation.

Also refactor audit cops into two "departments"
 - FormulaAudit
 - FormulaAuditStrict