[ci skip] Use const keyword to define CI_VERSION

Because.
diff --git a/system/core/CodeIgniter.php b/system/core/CodeIgniter.php
index 22072e9..804c685 100644
--- a/system/core/CodeIgniter.php
+++ b/system/core/CodeIgniter.php
@@ -55,7 +55,7 @@
  * @var	string
  *
  */
-	define('CI_VERSION', '3.1.1-dev');
+	const CI_VERSION = '3.1.1-dev';
 
 /*
  * ------------------------------------------------------