Fixing a problem of blinking a bubble.

Hello there

Thank you for making a nice cocoapod. I love it. I am using this on our project. :ok_woman: 

When I start playing a sound, the bubble always stats from 3:45. Because DKAudioPlay is initialized like that at https://github.com/wzbozon/DKAudioPlayer/blob/master/DKAudioPlayer/DKAudioPlayer.m#L148.  And then it moves to 0:00 by onTimer.

I guess It bothers if you want to change the sound.so I fixed a problem of it. It will always initialize the duration of sound which is set by initWithAudioFilePath. So It works nicely. What do you think?