chore(PHPUnit): fix travis build for php 5.6

Author: Jens Schulze

.travis.yml

@@ -35,7 +35,7 @@ before_script:
 - if [ $PHP != '5.4' ] ; then composer install -o --prefer-dist; fi
 script:
 - ant phpcs-ci
-- phpunit --testsuite=unit
+- vendor/bin/phpunit --testsuite=unit
 - if [ $PHP == '7.0' ] ; then phpunit --testsuite=integration; fi
 - ant behat
 after_success: