Allow PHPUnit from Composer

**Scenario**
When updating PHPUnit using homebrew it updated to `4.1.0` which turns out to not be compatible. As downgrading using homebrew is very hard, I decided to use Composer. So I created a `composer.json` and added `phpunit/phpunit 3.7.37` as a dependancy.

**Environment**
Cake 2.5.0

**Issue**
The issue is that the Test Suite Dispatcher doens't understand vendor/package style folders.

**Fix**
Just updated the dispatcher to look for a vendor/package style folder inside `Vendors` before trying the default.