DRY: Use constants for level codes instead of re-defining

src/Monolog/Logger.php

@@ -92,14 +92,14 @@ class Logger implements LoggerInterface
      * @var array $levels Logging levels
      */
     protected static $levels = array(
-        100 => 'DEBUG',
-        200 => 'INFO',
-        250 => 'NOTICE',
-        300 => 'WARNING',
-        400 => 'ERROR',
-        500 => 'CRITICAL',
-        550 => 'ALERT',
-        600 => 'EMERGENCY',
+        self::DEBUG     => 'DEBUG',
+        self::INFO      => 'INFO',
+        self::NOTICE    => 'NOTICE',
+        self::WARNING   => 'WARNING',
+        self::ERROR     => 'ERROR',
+        self::CRITICAL  => 'CRITICAL',
+        self::ALERT     => 'ALERT',
+        self::EMERGENCY => 'EMERGENCY',
     );
 
     /**