update Piwik to version 2.16 (fixes #91)

www/analytics/core/View/ViewInterface.php

@@ -1,6 +1,6 @@
 <?php
 /**
- * Piwik - Open source web analytics
+ * Piwik - free/libre analytics platform
  *
  * @link http://piwik.org
  * @license http://www.gnu.org/licenses/gpl-3.0.html GPL v3 or later
@@ -21,5 +21,5 @@ interface ViewInterface
      *
      * @return string Serialized data, eg, (image, array, html...).
      */
-    function render();
+    public function render();
 }