Fix layout bug when zooming page

When zooming the page, some of the controls get messed.
See [mediaelement.js volume control messed when increasing browser zoom](http://stackoverflow.com/questions/17272328/mediaelement-js-volume-control-messed-when-increasing-browser-zoom).

Simply decrese 1 pixel for `div.mejs-time-rail` can avoid the layout overflow.
Merge pull request #1097 from ChiChou/patch-1

Fix layout bug when zooming page