Use phpunit from vendor/bin dir

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

.travis.yml

@@ -97,7 +97,7 @@ script:
 
   # Run PHP tests
   - cd tests
-  - ../vendor/phpunit/phpunit/phpunit --configuration phpunit.xml --fail-on-warning
+  - ../vendor/bin/phpunit --configuration phpunit.xml --fail-on-warning
   - if [[ "$PHP_COVERAGE" = "TRUE" ]]; then wget https://scrutinizer-ci.com/ocular.phar;
     fi
   - if [[ "$PHP_COVERAGE" = "TRUE" ]]; then php ocular.phar code-coverage:upload --format=php-clover