Update responsive-video to play nice with flexbox and grid

[ashleykolodziej]

Right now, if you include a responsive video in a flexbox or grid layout, it will behave wildly differently between Chrome, Firefox, IE, and Safari. This is because none of these browsers deal with padding in quite the same way when it comes to flexbox and grid. See https://stackoverflow.com/questions/23717953/padding-bottom-top-in-flexbox-layout

The accepted answer there was to just use something besides padding, like height - but an answer further down moves the padding to a pseudo-element. This actually works pretty great and keeps the same aspect ratio. Updating this in our responsive video class will make it compatible with newer layouts.