Use PHPunit downloaded by composer

Author: ilijastuden

.travis.yml

@@ -4,8 +4,6 @@ php:
   - 7
 before_install:
   - composer self-update
-install:
-  - composer install --dev
-  - phpunit --self-upgrade
-script: phpunit
+install: composer install --dev
+script: vendor/bin/phpunit
 after_script: php ./vendor/bin/coveralls -v