Update GridFieldBulkImageUpload_Request.php

Silverstripe 3.1.0 requires statics to be defined as private

http://doc.silverstripe.org/framework/en/3.1/changelogs/3.1.0

code/GridFieldBulkImageUpload_Request.php

@@ -40,7 +40,7 @@ class GridFieldBulkImageUpload_Request extends RequestHandler {
 	/**
 	 *
 	 */
-	static $url_handlers = array(
+	private static $url_handlers = array(
 		'$Action!' => '$Action'
 	);
 	
@@ -430,4 +430,4 @@ class GridFieldBulkImageUpload_Request extends RequestHandler {
 			)));
 		return $items;
 	}
-}
+}