Update bootstrap.php

Composer does not give back `$loader` object when using `require_once` (returns `true` instead). Use require to get object.

The testing bootstrap does not support being started from within the vendor folder. Only run from the `src` folder is supported. The bootstrap could be more adaptive like this https://github.com/doctrine/doctrine2/blob/master/tests/Doctrine/Tests/TestInit.php

(overall i noticed the testing bootstrap is inconsistent throughout doctrine libraries)