Closes: caskroom/homebrew-cask#27075

Refactor Cask installer.

Fail if cask source already exists.

travis: use Xcode 8.1 on Sierra.

added "CLT" abbreviation after "Command Line Tools" in Installation.md doc

For code coverage from `brew test-bot`.

added 'CLT' abbreviation after 'Command Line Tools' (so that if someone follows link to footnote#3, the meaning of 'CLT' in footnote is clear)

bash_test: handle race condition.

Other tests may delete Bash files (e.g. `shcmd.sh`) while tests are
ongoing so ensure this doesn't cause a test failure.

fs_leak_logger: use more permissive mkpath.

tap-new: use latest Travis CI recommended config.

travis.yml: enable macOS builds again.

This avoids a race condition occurring here.

Pretty sure no-one cares about this except me but this works
consistently across a variety of different taps and is faster than the
old version.

uninstall_test: require testing_env

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.