Fix PHPUnit version to 3.7.35

"MediaWiki tries to require PHPUnit files directly,
but the phpunit/phpunit PEAR channel now falls back to the PHAR
method (install a single file, /usr/bin/phpunit, with all classes
packed inside it)." (from the bug report)

Bug: 64597
Change-Id: Ib93b32473ab1a96ea97d90c15fd3675e8ec9843c