Set root font size in px - Fixes layout issues in IE11

Fixes rendering issues on IE11.

See https://connect.microsoft.com/IE/feedback/details/816709/ie-11-calculating-font-sizes-wrong-when-setting-the-bodys-font-size-in-relative-units

>
If you set
```
body {
    font-size: 62.5%;
}
```
The calculated font size should be `10px`, but it is `9.93px` in IE11

Thanks @7studio and @dawitti

![ ](http://media.giphy.com/media/g8qNjmVyhNfOw/giphy.gif)