In 2.4 phpunit is specified as a 'require' package in the composer installer (composer.json), the 'require-dev' attribute, which is more appropriate for dev/test purposes can be used instead of it. Would be useful if in future Moodle intends to use composer to manage other libraries dependencies (used by Moodle core). The only difference using this attribute instead of 'require' is that composer has to be executed with --dev option (http://getcomposer.org/doc/04-schema.md#require-dev)