fix for android 4.1 browser

Hi,

Apparently, android 4.1 is weird in the way it defines the transition property : both transition and webkitTransition are defined, but only webkitTransitionEnd actually works. (See http://stackoverflow.com/questions/13823188/android-4-1-change-transition-and-webkittransition-defiend-how-to-properly-de)

By checking the webkit- prefix first, I managed to overcome this issue (at least on android 4.1).

Cheers