Log critical as critical, not exception.

This prevents weird things from happening when log.critical is called
in a non-except context.  Typically it would result in either 'None'
or the last exception being logged, neither of which is desirable.

gunicorn/glogging.py

@@ -53,7 +53,7 @@ class Logger(object):
 
 
     def critical(self, msg, *args, **kwargs):
-        self.error_log.exception(msg, *args, **kwargs)
+        self.error_log.critical(msg, *args, **kwargs)
 
     def error(self, msg, *args, **kwargs):
         self.error_log.error(msg, *args, **kwargs)