Updated require-dev to have phpunit

The documentation currently reflects using `require-dev` to run the test suite, but the example then (by inference) uses PEAR to run PHPUnit. 

Assuming someone is checking out Silex with `composer install --dev` to run tests, it would (in my opinion) be better to provide them with a PHPUnit as part of the install, also allowing to lock to version. They then can either use `vendor/bin/phpunit` to run the tests.