Pass /notice/server-environment/project-root.

According to Hoptoad API documentation this is
optional, but some third-party software using the
same API is more strict and errors out if we don't
provide it (Redmine, I'm looking at you).

lib/resque/failure/hoptoad.rb

@@ -109,6 +109,7 @@ def xml
           end
           x.tag!("server-environment") do
             x.tag!("environment-name",server_environment)
+            x.tag!("project-root", "RAILS_ROOT")
           end
 
         end