Use Application::getBootstrapFile() to get start.php path

https://github.com/laravel/framework/blob/master/src/Illuminate/Foundation/Application.php#L202-L210

The function exists, why not use it? This change will actually make it possible to require laravel/laravel in a package's `require-dev` and get access to the full default application stack.

My package which has controllers that I test using the default Laravel TestCase:

https://travis-ci.org/anlutro/laravel-4-core https://github.com/anlutro/laravel-4-core/tree/develop