Update travis phpunit

Author: sebastianfeldmann

.travis.yml

@@ -8,7 +8,7 @@ php:
 before_script:
   - composer self-update
   - composer install --no-interaction --prefer-source
-  - wget -O phpunit.phar https://phar.phpunit.de/phpunit-7.5.9.phar
+  - wget -O phpunit.phar https://phar.phpunit.de/phpunit-8.5.3.phar
   - if [ $TRAVIS_PHP_VERSION = "7.4" ]; then wget -O phpstan.phar https://github.com/phpstan/phpstan/releases/download/0.10.5/phpstan.phar ; fi
 
 script: