Use cURL to fetch feeds

Previously, used SimpleXMLElement's ability to fetch XML from a URL;
however some feeds are now returning 403 headers if no user agent is
present.

This patch uses curl to fetch the feed XML. This is more robust, as it
also allows us to follow redirects properly.
Require cURL extension in order to operate
Merge branch 'hotfix/http-user-agent'

Fixes an issue with feeds that require a valid User-Agent header in order to
succeed.