Update User.pm - ensure numeric value for day

The XML must have changed at some point - join date looks like:

<memberSince>December 8th, 2007</memberSince>

This results in errors with the current module:

The 'day' parameter ("8th") to DateTime::new did not pass the 'an integer which is a possible valid day of month' callback
 at /Library/Perl/5.12/darwin-thread-multi-2level/DateTime.pm line 199.