Use Rails environment in /stats

Use the `Rails.env` instead of `Sinatra::Application.environment`.

Sinatra depends on `APP_ENV` rather than `RAILS_ENV` and defaults to
`development` if no `APP_ENV` is set. This can lead to situations where
the host application is running in `production` but the dashboard is
showing `development`.

Fixes ejschmitt#112

lib/delayed_job_web/application/views/stats.erb

@@ -5,7 +5,7 @@
       environment
     </th>
     <th>
-      <%=h Sinatra::Application.environment.to_s %>
+      <%=h Rails.env.to_s %>
     </th>
   </tr>
   <tr>