Check for cancelAnimationFrame when we call it.

stopCheckChange() is called twice, and in
[one](https://github.com/ginader/HTML5-placeholder-polyfill/blob/5805e4d11cd8790cf0af422fb487df5a61dc81e3/src/placeholder_polyfill.jquery.js#L56)
instance, it isn't checking for window.cancelAnimationFrame first.
Merge pull request #58 from samuelcole/master

Check for `cancelAnimationFrame` when we call it.