* Causes a bug in Formula#installed_alias_target_changed? when
  Formula#superseds_an_installed_formula? returns true because
  Formula#old_installed_formulae includes f for some Formula f.

* Causes a bug when foo@2.4 with alias foo has HEAD or devel version and
  we try to `brew upgrade foo --devel|--HEAD` from stable. The upgrade fails
  while since we installing formula to the same prefix it's alredy installed.
  The reason for that is that we use
  `formula_to_install = outdated.map(&:latest_formula)` in cmd/upgrade
  before calling upgrade_formula on foo.

  ```ruby
  def latest_formula
  installed_alias_target_changed? ? current_installed_alias_target : self
  end
  ```

  Formula#installed_alias_target_changed? compares formulae using
  Formula#==, which is wrong for this case, thus Formula#latest_formula doesn't
  return self and returns Formula#current_installed_alias_target with spec
  foo was initially installed instead of devel or HEAD, causing the error.

Small improvement to the xcode license hint. Reduces friction by allowing user to accept the license in a single step. 
Reference: http://apple.stackexchange.com/questions/175069/how-to-accept-xcode-license

Handle the case when an argument passed to `brew cask` is both the name
of a Cask and a non-Cask file in the current working directory.

Fixes https://github.com/caskroom/homebrew-cask/issues/25611.

This is handled by `brew style` now.

- unset IFS to fix version parsing on older Bashes
- reuse existing HOMEBREW_MACOS_VERSION if it’s available
- set rather than unset popup_stub

On older versions of OS X (i.e. pre-Mavericks) `xcode-select` is not
required to be set when using the CLT. Fix this behaviour so `brew
update` doesn't tell everyone on that configuration to `brew install
git`.

Mountain Lion: http://opensource.apple.com/source/libarchive/libarchive-25.1/config.h
  `/* #undef HAVE_LIBLZMA */`

Mavericks: http://opensource.apple.com/source/libarchive/libarchive-29/config.h
  `#define HAVE_LIBLZMA 1`

Sometimes `file` output contains data from the file under examination,
which may include binary data that does not represent valid UTF-8
codepoints. String#split dies if it doesn't understand the encoding, so
tell Ruby to treat `file` output as a bytestring.

Avoid using an unnecessary `xz` dependency when it's not needed.

Write the README but also a `.travis.yml` file (and in future perhaps a
`Jenkinsfile`).

It looks like a rebase/merge conflict remmenet.

Multiple issues have been filed about this environment variable, hence
give a warning. #932 might make this unnecessary when merged, but
hopefully this warning can be merged quickly.