warn depend_on :macos on linux platform

before

~~~
$ brew install passenger
passenger: OS X Mountain Lion or newer is required.
Error: An unsatisfied requirement failed this build.
~~~

after
~~~
$ brew install passenger
passenger: OS X Mountain Lion or newer is required.
==> Downloading http://s3.amazonaws.com/phusion-passenger/releases/passenger-4.0.45.tar.gz
Already downloaded: /var/cache/Homebrew/passenger-4.0.45.tar.gz
==> /usr/local/var/rbenv/versions/2.1.1/bin/rake apache2
~~~