Construct build result URL from parts

Currently, there is no easy way to grab this information from
the Build model.
So we need to construct it from various parts at our disposal.

lib/travis/api/app/responders/atom.rb

@@ -16,7 +16,7 @@ module Travis::Api::App::Responders
   <% @builds.each  do |build| %>
   <entry>
     <title><%= build.repository.slug %> Build #<%= build.number %></title>
-    <link href="<%= endpoint.url %>" />
+    <link href="<%= File.join("https://", Travis.config.host, build.repository.slug, "builds", build.id.to_s) %>" />
     <id>urn:uuid:<%= SecureRandom.uuid %></id>
     <updated><%= build.finished_at || build.started_at %></updated>
     <summary type="html">