Update justified-nav.css (Removed unnecessary ms-linear-gradient prefix)

Removed unnecessary ms-linear-gradient prefix. There was never a stable release of IE that supported -ms- prefixed gradients, those were only in preview versions (stable IE10 supports both prefixed and unprefixed gradients). (source: http://lea.verou.me/2013/04/can-we-get-rid-of-gradient-prefixes/)