Animated transition on `window.history.back()`

~~Animated back transitions are working when using push.js via data attributes (apparently, though I never checked). This pr makes it so that they also work when using push.js via PUSH() function in javascript.~~

Animated back transitions don't work at all. Demo (enable touch emulation in devtools):
http://push-js-test.s3.amazonaws.com/test.html

This pr fixes it.