[dev] use skin name constant instead of string

Use centralized symbol for "vector" instead of hardcoded string.

Change-Id: I44d6b3e64cd424b6624ce5c050616e5bb281fbc8

includes/SkinVector.php

@@ -32,7 +32,7 @@ use Vector\Constants;
  * @unstable
  */
 class SkinVector extends SkinTemplate {
-	public $skinname = 'vector';
+	public $skinname = Constants::SKIN_NAME;
 	public $stylename = 'Vector';
 	public $template = 'VectorTemplate';