New version of homebrew passenger

* Upgraded/Rewrote Homebrew Passenger to use `up` for setup, install and update #23
* Added a test suite for the Homebrew Passenger #33
  * It won't notice if you swop the order of installing homebrew, installing packages and updating packages. This is possible but would make the test really awful. I think that's not worth.
* Added two additional executor functions:
    * `command_exists?`: Check if a shell command is available
    * `silent_execute`: Execute a shell command without any output