posts: fix incorrect duration shown for certain videos.

Fix thumbnails incorrectly showing a duration of "0:00" when a video had
a duration between 59.5 seconds and 60.0 seconds. This happened because
of incorrect rounding - the seconds value was rounded up, but the
minutes value wasn't.

Examples: https://danbooru.donmai.us/posts?tags=duration:59.5...60.0+status:any

app/helpers/application_helper.rb

@@ -103,9 +103,10 @@ module ApplicationHelper
   end
 
   def duration_to_hhmmss(seconds)
+    seconds = seconds.round
     hh = seconds.div(1.hour).to_s
     mm = seconds.div(1.minute).to_s
-    ss = "%.2d" % (seconds.round % 1.minute)
+    ss = "%.2d" % (seconds % 1.minute)
 
     if seconds >= 1.hour
       "#{hh}:#{mm}:#{ss}"