[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to do the same thing, but became outdated.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/Migration.php b/system/libraries/Migration.php
index f46d45f..a2dce13 100644
--- a/system/libraries/Migration.php
+++ b/system/libraries/Migration.php
@@ -126,7 +126,7 @@
 			$this->{'_'.$key} = $val;
 		}
 
-		log_message('debug', 'Migrations class initialized');
+		log_message('info', 'Migrations Class Initialized');
 
 		// Are they trying to use migrations while it is disabled?
 		if ($this->_migration_enabled !== TRUE)