Update justified-nav.css

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/)
Merge pull request #9973 from coliff/patch-6

Update justified-nav.css (Removed unnecessary ms-linear-gradient prefix)
change `else if` to `else`
Merge pull request #9987 from wangsai/fix-typo

change `else if` to `else`
Fixes #9974

Added th selector in .table-row-variant mixin
Merge pull request #9999 from sqrt4/fix-9974

Fixes #9974
remove .btn-group

Removing this class--it's not used in the example and using it causes "bad things to happen." 

http://jsfiddle.net/josephtate/Jn6H4/
Merge pull request #10006 from jodytate/master

remove .btn-group
Update Grunt instruction links and wording
Merge pull request #10011 from nschonni/patch-1

Update Grunt instruction links and wording