[#521] Fix regexp for calculating top position with useTransform:false

Fixes this issue: https://github.com/cubiq/iscroll/issues/521

Updated fiddle: http://jsfiddle.net/5eYMZ/1/

Problem with old regexp was that it removes dot's, i.e.:

```
+"-1500.1531982421875px".replace(/[^-\d]/g, '')
-15001531982421875
```