PHPStan: Use stable PHPUnit path

phpunit-bridge will create a symlink.

(cherry picked from commit 2c6c6d5)

Author: jtojnar

phpstan.neon

@@ -6,7 +6,7 @@ parameters:
 
     # https://github.com/phpstan/phpstan/issues/694#issuecomment-350724288
     bootstrapFiles:
-        - vendor/bin/.phpunit/phpunit-7.5-0/vendor/autoload.php
+        - vendor/bin/.phpunit/phpunit/vendor/autoload.php
 
 includes:
     - vendor/phpstan/phpstan-phpunit/extension.neon