Fix task UI to display proper change url

Gerrit.url is not available post 2.16, thus stop using it.

Change-Id: Ibaef74deb495245ae87fac83c54be3d46f8bb9f7

gr-task-plugin/gr-task-plugin-tasks.js

diff --git a/gr-task-plugin/gr-task-plugin-tasks.js b/gr-task-plugin/gr-task-plugin-tasks.js
index c4432c3..7653ed4 100644
--- a/gr-task-plugin/gr-task-plugin-tasks.js
+++ b/gr-task-plugin/gr-task-plugin-tasks.js
@@ -51,7 +51,7 @@
   }
 
   _getChangeUrl(change) {
-    return Gerrit.url(change.toString());
+    return '/c/' + change.toString();
   }
 }