x11_requirement: move download to base class.

formula: sort aliases in to_hash.

No reason to have it in macOS class and keeps consistent output with
`brew info` on Linux.

Otherwise the ordering varies based on the filesystem.

requirements: define cask on base class.

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.