AppVeyor - adjust phpunit version

appveyor.yml

@@ -23,7 +23,7 @@ install:
     - echo extension=php_openssl.dll >> php.ini
     - cd C:\tools
     - appveyor DownloadFile https://getcomposer.org/composer.phar
-    - appveyor DownloadFile https://phar.phpunit.de/phpunit.phar
+    - appveyor DownloadFile https://phar.phpunit.de/phpunit-5.7.phar
     - git config --global github.accesstoken 5e7538aa415005c606ea68de2bbbade0409b4b8c
 
 before_test:
@@ -32,5 +32,5 @@ before_test:
 
 test_script:
     - cd C:\projects\php-cs-fixer
-    - php C:\tools\phpunit.phar --verbose
+    - php C:\tools\phpunit-5.7.phar --verbose
     - php php-cs-fixer --diff --dry-run -v fix