Tweaked progress bar animation

Before:
![before](https://cloud.githubusercontent.com/assets/162986/4667182/5c288312-555a-11e4-848f-ffd6e278a18c.gif)

After:
![after](https://cloud.githubusercontent.com/assets/162986/4667184/600e8986-555a-11e4-89aa-e7ea9a62781e.gif)