Move drush from main homebrew to homebrew-php

 * Moved drush from main homebrew to homebrew-php.

   The drush formula now requires a build dependency on composer which is
   located in homebrew-php.

 * Drush HEAD build depends on composer.
    
   The HEAD version of drush has a build dependency on composer.

See Homebrew/homebrew#31696 and Homebrew/homebrew#30596 for issues regarding not having the formula in the main Homebrew repository anymore.

Ping; @mikemcquaid.
I'll try to come up with something reasonable.

There is a large PHPUnit test suite for drush but I guess it is overkill to run that?

If I were to run the test suite it would be to be able to do something like this:
```ruby
depends_on "Homebrew/php/phpunit" => :test
```