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os/mac: mark Mavericks as outdated.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.

Make `HOMEBREW_PREFIX/Caskroom` the default.

Make `brew upgrade` and `brew outdated` follow alias changes

brew.sh: fix repository auto-update change.

On auto-update `HOMEBREW_LIBRARY` may change location which means that
it won't be found for the actual install command. Look for this having
occurred and then set the new `HOMEBREW_LIBRARY` (and
`HOMEBREW_REPOSITORY`) accordingly.

More Rubocop tweaks