Stop showing progress bar on every push state change

# What's up

In my experience, the progress bar was sluggish, since most of the times it was starting after the page change had already been rendered. The plugin we are using is [pace](http://github.hubspot.com/pace/)

So I dug inside the pace documentation and discovered that by default they enable the progress bar on every pushState event: since Ember changes the url on a lot of actions we had a lot of unnecessary progress bar movements.

This PR disables the automatic pushState from pace :frog: 

# Bonus

Since pace does everything *automagically*, we don't need to set it in the LoadingRoute: it will react to slow ajax calls and event lags by itself.