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
Could you make it more descriptive? Please write that it is needed only for Safari and the reason why it do not work only in this browser (I think due to inconsistent events triggering).