[tests] use phpunit from dependencies instead of global install

Signed-off-by: Dorian Savina <[email protected]>

Author: dsavina

.travis.yml

@@ -9,7 +9,7 @@ before_script:
  - composer install --no-interaction
 
 script:
- - phpunit
+ - vendor/bin/phpunit
 
 after_script:
  - php vendor/bin/coveralls -v