generate dartisans from playlist, keep everything up to date

src/site/dartisans/dartisans-audio.xml

 ---
 layout: none
 ---
 <?xml version="1.0" encoding="utf-8"?> 
 <rss version="2.0" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd"> 
 <channel>
   <title>Dartisans: Dart Programming Language Podcast</title>
   <link>http://www.dartlang.org/dartisans/</link> 
   <description>News, interviews, and Q&amp;A with the Dart team and community.</description>
   <itunes:summary>News, interviews, and Q&amp;A with the Dart team and community.</itunes:summary> 
   <language>en</language> 
   <copyright>This work is licensed under a Creative Commons Attribution 3.0 Unported License.</copyright> 
   <lastBuildDate>{{ "now" | date_to_rfc2822 }}</lastBuildDate>
   <itunes:image href="http://www.dartlang.org/dartisans/dartisans-podcast.png" />
   <itunes:author>Dart Team</itunes:author>
   <itunes:keywords>dart,web,programming,javascript,html5</itunes:keywords>
   <itunes:explicit>No</itunes:explicit>
   <itunes:category text="Technology">
     <itunes:category text="Software How-To" />
   </itunes:category>
   {% injectdata episodes dartisans/episodes.yaml %}
   {% for episode in page.episodes.episodes %}
+  {% if episode.file %}
   <item>
     <title>{{ episode.title }}</title>
     <description>{{ episode.description | strip_html }}</description>
     <pubDate>{{ episode.pubdate | date_to_rfc2822 }}</pubDate>
     <guid>{{ page.episodes.url-prefix }}{{ episode.file }}</guid>
     <enclosure url="{{ page.episodes.url-prefix }}{{ episode.file }}" length="{{ episode.length }}" type="audio/mpeg" />
   </item>
+  {% endif %}
   {% endfor %}
 </channel> 
 </rss>