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os/mac: mark Mavericks as outdated.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Make `brew upgrade` and `brew outdated` follow alias changes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

Closes https://github.com/Homebrew/homebrew-core/issues/4973

References #1023.

Can't see how this could ever happen but let's safe-guard it just in
case (as ilovezfs saw it).

update-report: migrate everyone to new repository.

Use new "macOS" naming where appropriate.

update-report: remove cask dir after migration.

keg: only prune var/homebrew/linked.

Relocate HOMEBREW_REPOSITORY when necessary.

This will make brew readall --syntax pass