Fix state.playing.jumpToTime behavior (#672)

Previously, state.playing.jumpToTime = 0 didn't do anything.
This commit fixes that.

renderer/views/player.js

@@ -43,7 +43,7 @@ function renderMedia (state) {
       mediaElement.play()
     }
     // When the user clicks or drags on the progress bar, jump to that position
-    if (state.playing.jumpToTime) {
+    if (state.playing.jumpToTime != null) {
       mediaElement.currentTime = state.playing.jumpToTime
       state.playing.jumpToTime = null
     }