It looks like Google has changed their RSS feeds to redirect HTTP traffic to HTTPS. If you look at the response of the the XML file you're trying to fetch it actually returns HTML with redirect tags.
Simply call the https directly to get around this.
$rss = new SimpleXMLElement('https://groups.google.com/group/boy-scout-troop-845/feed/rss_v2_0_msgs.xml', null, true);