Symfony deprecations #87

Method "Symfony\Component\DependencyInjection\Extension\Extension::getAlias()" might add "string" as a native return type declaration in the future. Do the same in child class "Exercise\HTMLPurifierBundle\DependencyInjection\ExerciseHTMLPurifierExtension" now to avoid errors or add an explicit @return annotation to suppress this message.

Method "Symfony\Component\Config\Definition\ConfigurationInterface::getConfigTreeBuilder()" might add "TreeBuilder" as a native return type declaration in the future. Do the same in implementation "Exercise\HTMLPurifierBundle\DependencyInjection\Configuration" now to avoid errors or add an explicit @return annotation to suppress this message.

Method "Symfony\Component\HttpKernel\CacheWarmer\WarmableInterface::warmUp()" might add "array" as a native return type declaration in the future. Do the same in implementation "Exercise\HTMLPurifierBundle\CacheWarmer\SerializerCacheWarmer" now to avoid errors or add an explicit @return annotation to suppress this message.