fix #9924
fix #9917
CONTRIBUTING: reflect move away from *-wip branch style of development

Per http://blog.getbootstrap.com/2012/12/10/bootstrap-3-plans/ :
> * Drop the `*-wip` branch style of development.
point release download link to an actual release instead of `master`
links to a release instead of master, so rephrase description accordingly
master branch is no longer stationary, so don't imply otherwise
fix #9915
update copyright year
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)