Fixed transcript not follow with video on seek.

TNL-235

If video player is in playing state then in ```seekTo()``` timer was stopped to call ```update()``` at regular interval of time here [03_video_player.js#L466-L467!](https://github.com/edx/edx-platform/blob/master/common/lib/xmodule/xmodule/js/src/video/03_video_player.js#L466-L467) 
but not triggering it again to call ```update()``` at regular interval of time.

According to my thinking this problem will occur in all browsers.

Ticket: https://openedx.atlassian.net/browse/TNL-235
@polesye I think its not a Safari issue, i tested it in chrome and its not working in it too.