[ci skip] Change some log messages' level

'Class Loaded' type of messages flood log files when
log_threshold is set to 2 (debug). They're now logged
as 'info' level.

This is manually applying PR #1528, which was created
to do the same thing, but became outdated.
diff --git a/system/libraries/Session/Session.php b/system/libraries/Session/Session.php
index 5c61002..bc4a5ca 100644
--- a/system/libraries/Session/Session.php
+++ b/system/libraries/Session/Session.php
@@ -174,7 +174,7 @@
 
 		$this->_ci_init_vars();
 
-		log_message('debug', "Session: Class initialized using '".$this->_driver."' driver.");
+		log_message('info', "Session: Class initialized using '".$this->_driver."' driver.");
 	}
 
 	// ------------------------------------------------------------------------