Implement Executor#command_exists?

Inspired by [thoughtbot's
laptop](https://github.com/thoughtbot/laptop/blob/master/mac)
Added Executor#silent_executor

This is super simple, but it is important for testing the passengers.
We can mock this easily, compared to backticks.
Adjusted Homebrew to #23
Added Test Suite for Homebrew #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.
Merge pull request #35 from moonglum/homebrew-passenger

New version of homebrew passenger
Merge branch 'master' of https://github.com/moonglum/exogenesis