Update grunt-contrib-uglify to 0.6.0

Upstream changes https://github.com/gruntjs/grunt-contrib-uglify/compare/v0.5.0...v0.6.0

see #30141


git-svn-id: https://develop.svn.wordpress.org/trunk@30066 602fd350-edb4-49c9-b593-d223f7449a82