Fixes #6829 : fix coveralls error calls comes by phpcov

- [x] Change <code>sebastianbergmann/phpcov</code> version 2.0.* to <code>phpunit/phpcov</code> rely on packagist
- [x] Update phpcov command line argument to merge <code>build/coverage/*</code>
- [x] <code>phpunit/phpcov</code> use phpunit version 4.3.4, so I removd <code>phpunit/PHPUnit</code> from require-dev.
- [x] Updated docs and test Bootstrap.php to require phpunit version 4.3.4