Fix for Firefox post version 16

Following the advice posted here:

https://developer.mozilla.org/en-US/docs/CSS/Media_queries#-moz-device-pixel-ratio

and here:

http://www.w3.org/blog/CSS/2012/06/14/unprefix-webkit-device-pixel-ratio/

I've added the `min-resolution` declaration to the media query to make it work with modern Firefox editions.