* installing HEAD keg_only should be possible only if:
  1.
    - Old version installed differs from new one and optlinked
    - `--force` flag is passed
    - HEAD is seriously outdated or outdated with `--fetch-HEAD`
  or 2.
    - Old version installed differs from new one and not optlinked
  or 3.
    - This formula is not installed

* installing keg_only should be possible only if:
  1.
    - Old version installed differs from new one and optlinked
    - `--force` flag is passed
  or 2.
    - Old version installed differs from new one and not optlinked
  or 3.
    - This formula is not installed

* installing HEAD should be possible only if:
  1.
    - Old HEAD is seriously outdated or outdated with `--fetch-HEAD`
  or 2.
    - HEAD is not installed

* installing stable or devel should be possible only if:
  - Old version installed differs from new one

* `Formula#linked?` returns true if formula linked
* `Formula#optlinked?` returns true if formula linked to opt
  formula installed to the Cellar
* `Formula#prefix_linked?` returns true if linked keg points to the prefix
  passed as an argument
* `Formula#linked_version` to get linked version of the formula

update.sh: Use full path to pkill

Fixes #1600.

Fix macOS version comparison

docs/Checksum_Deprecation.md: Note that SHA-1 now blocks installation

dependency_collector: Make ant_dep and xz_dep public

They were already public on macOS, but they were made
private by mistake on all other platforms.
DependencyCollector.tar_needs_xz_dependency? depends
on xz_dep being public, so there's no turning back now :(

rubocop: exclude more files.

tap-new: cleanup and update Travis configuration.

We never want to check these for any e.g. taps that use them.

uses_test: don't rely on output order.

This document was out of date as of https://github.com/Homebrew/brew/pull/1451

diagnostic: silence a Jenkins doctor warning.

Allow our Jenkins CI testing for Homebrew/brew to live outside of
`/usr/local` without complaint (as it's not using any binary packages
anyway).

- Use macOS Sierra 10.12 and Xcode 8.
- Add indentation.

I've seen this vary.

update-report: allow skipping analytics message.

This is used by the install script so it can simplify it's logic and
make the analytics messaging more prominent.

update-report: further tweak analytics output.

diagnostic: silence outdated Travis Xcode warning.

There were some bugs with the use of `HOMEBREW_NO_ANALYTICS_THIS_RUN`
and some output glitches I missed with the previous PR (#1575).

update-report: further improve analytics messaging

This doesn't add any value and stops us from testing `brew doctor` and
others from running `brew doctor` on Travis CI.

- Don't output the analytics message and mark it as read if
`HOMEBREW_NO_ANALYTICS_THIS_RUN` is set. This mostly simplifies the
installer code where we can display the message ourselves there rather
than having `brew update` print it out sometimes and not others (i.e.
when there's no system Git installed).
- Use the shell's audible bell to nudge people to actually read this
message (and give them less excuse to complain when they don't).
- Add an extra newline and bold all the analytics messaging so it's
more visible in the output.

Change my site address

I no longer maintain my former site so it's better to link to my GitHub account.

Revert "Fix early exit condition for check_for_member_of_admin_group"

This reverts commit 7f13b37b, reversing
changes made to 62e0ecda.

This reverts commit 89c33489, reversing
changes made to 7f13b37b.

Fix early exit condition for check_for_member_of_admin_group

diagnostic: check if the user is a member of the admin group.

exceptions: better handle unsupported macOS versions.

Non-administrative users are unable to run `brew linkapps` (and possibly other
commands) under normal circumstances.