use interface_exists instead of class_exists

Author: kbond

Command/ServerLogCommand.php

@@ -37,7 +37,7 @@ public function isEnabled()
  }
 
  // based on a symfony/symfony package, it crashes due a missing FormatterInterface from monolog/monolog
- if (!class_exists(FormatterInterface::class)) {
+ if (!interface_exists(FormatterInterface::class)) {
  return false;
  }