[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to do the same thing, but became outdated.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/Trackback.php b/system/libraries/Trackback.php
index 3284923..9992fb4 100644
--- a/system/libraries/Trackback.php
+++ b/system/libraries/Trackback.php
@@ -103,7 +103,7 @@
 	 */
 	public function __construct()
 	{
-		log_message('debug', 'Trackback Class Initialized');
+		log_message('info', 'Trackback Class Initialized');
 	}
 
 	// --------------------------------------------------------------------