Fix webm converter outputting non-webm videos

mkvmerge didn't realize it was supposed to output a webm specifically
because write_path does not have a file extension. The --webm option
tells it this instead.

app/logical/pixiv_ugoira_converter.rb

@@ -72,7 +72,7 @@ class PixivUgoiraConverter
 
       ext = folder.first.name.match(/\.(\w{,4})$/)[1]
       system("ffmpeg -i #{tmpdir}/images/%06d.#{ext} -codec:v libvpx -crf 4 -b:v 5000k -an #{tmpdir}/tmp.webm")
-      system("mkvmerge -o #{write_path} --timecodes 0:#{tmpdir}/timecodes.tc #{tmpdir}/tmp.webm")
+      system("mkvmerge -o #{write_path} --webm --timecodes 0:#{tmpdir}/timecodes.tc #{tmpdir}/tmp.webm")
     end      
   end