Add fix for iOS8 Safari

This diff addresses the iOS8 Safari issues mentioned in sole/tween.js/issues/145 (specifically [this comment](https://github.com/sole/tween.js/issues/145#issuecomment-62250157))

(I hope the style is sufficiently close to the rest of the code.  As you can see I opted not to add this into the existing ternary, as that seemed like it wouldn't be very readable!)

Compare [my forked gh-pages branch](http://lochlan.github.io/tween.js/examples/01_bars.html) versus [your current gh-pages branch](http://sole.github.io/tween.js/examples/01_bars.html) using Safari on iOS8.