Wrapper-type fix for <progress> element not returning to an indeterminate state (#1431)

Here's a wrapper solution to #1431, in case it is decided that a [wrapper](https://github.com/facebook/react/pull/1437#issuecomment-41177553) is the way to go, rather than a [mutation method](https://github.com/facebook/react/pull/1437#issuecomment-41199618).