[+]: use "symfony/var-exporter" for OpCache v2.1

-> better performance, because we don't need to serialize the data

.travis.yml

@@ -47,6 +47,7 @@ before_script:
   - travis_retry composer require satooshi/php-coveralls:1.0.0
   - travis_retry composer require predis/predis
   - if [ "$(phpenv version-name)" == 7.3 ]; then travis_retry composer require phpstan/phpstan; fi
+  - if [ "$(phpenv version-name)" == 7.3 ]; then travis_retry composer require symfony/var-exporter; fi
   - travis_retry composer install --no-interaction --prefer-source
   - composer dump-autoload -o