Log Message:
Update get_iplayer to 3.13

Many fixes including:
 * Restored cache updating, which was broken by changes to BBC web
   sites. If you find search results missing programmes from the week
   of 19 Feb, rebuild your cache with --rebuild-cache to fill any
   holes.
 * Fixed a bug that generated incorrect schedule URLs (used for cache
   refresh) for the first calendar week of 2018 (and some future
   years). (@welwood08)