[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to do the same thing, but became outdated.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/Cart.php b/system/libraries/Cart.php
index a0fe105..2fffd9a 100644
--- a/system/libraries/Cart.php
+++ b/system/libraries/Cart.php
@@ -115,7 +115,7 @@
 			$this->_cart_contents = array('cart_total' => 0, 'total_items' => 0);
 		}
 
-		log_message('debug', 'Cart Class Initialized');
+		log_message('info', 'Cart Class Initialized');
 	}
 
 	// --------------------------------------------------------------------