Add version check for Vector

Sometimes, someone tries to install git master version of Vector on
a stable version of MediaWiki, which can result in a PHP error, e.g.
because of the missing status indicators in MW 1.24.

Add a version check to Vector.php to check, if a minimum mediawiki version
(actual 1.25) is installed in order to use this version.

Change-Id: I10f071253e48e6492949ebbc3c07964c747e885f

Vector.php

@@ -22,6 +22,12 @@
  * @ingroup Skins
  */
 
+if ( version_compare( $wgVersion, '1.25c', '<' ) ) {
+	echo 'This version of the Vector skin requires at least MediaWiki 1.25, you have ' . $wgVersion .
+		'. You can download a more appropriate version here: https://www.mediawiki.org/wiki/Special:SkinDistributor/Vector';
+	exit();
+}
+
 $GLOBALS['wgExtensionCredits']['skin'][] = array(
 	'path' => __FILE__,
 	'name' => 'Vector',