Homebrew/homebrew — pull request #31696 by arnested (2014-08-19 10:58:15+00:00)
Title: Drush HEAD build depends on composer.

The HEAD version of drush has a build dependency on composer from the
Homebrew/php tap.

Because of the dependency it might make sense to move the formula to Homebrew/php eventually (once HEAD reaches an official release, maybe).