Fixes flicker when progress animation completes

This fixes the flicker seen when the progress animation completes, as mentioned in issue #30. The way your example project is currently setup, you don't actually see the flicker. Here's how you can test it:

I'm using iOS7 base SDK and iPhone Retina (4-inch 64-bit) simulator. In `ViewController.m` comment out `[self startAnimation];` on line 53 and add in `[self.progressView setProgress:0.7 animated:YES];`.

Run the app and notice the flicker when the animation completes. Merge this branch and confirm the flicker is fixed.