add favicon

app/views/layouts/application.html.slim

@@ -9,6 +9,7 @@ html lang="ja"
     link href="//cdnjs.cloudflare.com/ajax/libs/fullcalendar/1.6.4/fullcalendar.css" rel="stylesheet" /
     link href="//cdnjs.cloudflare.com/ajax/libs/fullcalendar/1.6.4/fullcalendar.print.css" rel="stylesheet" /
     link href="//netdna.bootstrapcdn.com/font-awesome/4.0.3/css/font-awesome.css" rel="stylesheet"
+    = favicon_link_tag 'favicon.ico'
     script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"
     = render partial: 'partials/google_analytics'
     = stylesheet_link_tag    "application", media: "all"