update.sh: update to latest Homebrew/brew tag.

To test the tag update functionality allow setting
`HOMEBREW_UPDATE_TO_TAG`.

Rather than following every change on `master` let’s have non-developer
users (i.e. those who have never run a `dev-cmd` or set
`HOMEBREW_DEVELOPER`) update between tags.

This provides a fairly natural beta (the `master` branch`) and stable
(the tags) approach without restricting us to any particular way of
managing our tags.

PULL_REQUEST_TEMPLATE.md: Fix CONTRIBUTING.md link

Update: ensure tags are fetched

Update `brew-cask` man page and simplify `brew man`.

We use these for updating people who just follow tags.

Closes #1040.

Rearrange repository

Readme tweaks

update-report: migrate on newer auto-update.

If we have a `brew.sh` which has set
`HOMEBREW_ENABLE_AUTO_UPDATE_MIGRATION` then let's allow an auto-update
migration. That's because it contains the fix below it _before_ the
update happened which means the auto-update won't fail in the same way
as if updating from an old version.

test-bot: always set PATH to HOMEBREW_PREFIX.

Set the `PATH` to always have the `bin` and `sbin` for the current
`HOMEBREW_PREFIX` at the beginning.

os: update Linux troubleshooting url

No auto update migration

os/mac: mark Mavericks as outdated.

Use user GEM_HOME for installing gems

Save runtime dependencies in INSTALL_RECEIPT.json

Fixes #930

Save the value of GEM_HOME before unsetting it,
and resetting it when setting up a gem environment.

Fixes #944.