Update web_ui.py

With plug-in Agilo(1.3.9-PRO) installed, trac.ticket.web_ui.TicketModule will be disabled by Agilo automatically. Instead, agilo.ticket.web_ui.AgiloTicketModule will be enabled, this will cause the changelogs of TracTicketChangelogPlugin can not be displayed in ticket view.

This change is a workaround for the issue above, there maybe some solution more properly I don't know. If anyone knows please tell.