The `cask` attribute doesn't make as much sense on Linux but can be
ignored there. The advantage of this change is that (like #4086) it
allows figuring out the relevant cask for a formulae requirement on a
Linux machine.

formula: tweak to_hash bottle output.

AbstractDownloadStrategy#safe_system : Don't splat 2nd arg to ErrorDuringExecution::new

Make some changes required to have `brew info --json=v1` whether there
is any bottle block rather than a bottle checksum for the given system.

This provides more consistent output.

Xcode 9.3

Fixes #4084.

formula_support: deprecate more keg_only reasons.

It's unnecessary extra complexity to have versions that are keg-only on
some versions of macOS and not others.

Initially this was to only do so on old versions of OS X and Xcode but
the discussion in https://github.com/Homebrew/brew/pull/4081 meant that
it made more sense to remove them all.

Remove Xcode.installed? guards.

RuboCop 0.55.0.

It's safe since 600d58f4 when
MacOS::Xcode.version started returning `Version::NULL` where
appropriate.

Cask Refactoring

missing: exit nonzero if missing deps are found

audit: new formulae should not have a HEAD spec

Update `rubocop-cask` to 0.18.1.

Add missing `MacOS.release` deprecation.

cli_parser: Refactor required argument for flag to '='

docs/Analytics: note retention period.

Due to [GDPR](https://www.eugdpr.org) Google Analytics have added
[data retention controls](https://support.google.com/analytics/answer/7667196).

Let's set these so that we don't keep any anonymous user data any longer
than we need it.

compat/extend/string: fix odisabled return.

`odisabled` will still return for a formula in `.brew`. This means
`EOS.undent` returns `nil` and the formula cannot be parsed. Instead
return the actual string in this case to avoid e.g. patches blowing up
with `nil` strings.

Fixes #4049.

Fix Cask search not showing remote matches.

Make release-notes command work from any location