Fix: Use phpunit as installed with Composer

Author: localheinz

.travis.yml

@@ -19,6 +19,9 @@ before_install:
 install:
   - composer install --prefer-dist
 
+script:
+  - vendor/bin/phpunit
+
 before_script:
   - git config --global user.email "[email protected]"
   - git config --global user.name "Travis CI"