Bump grunt-contrib-uglify to ~0.6.0

Otherwise, Grunt will not work.

    Running "uglify:dist" (uglify) task
    Warning: Unable to write "true" file (Error code: undefined). Use --force to continue.

Apparently, that's because the Gruntfile.js is using the [newer version of configuration](https://github.com/gruntjs/grunt-contrib-uglify#migrating-from-2x-to-3x) than that specified in `package.json`.

Upgrading grunt-contrib-uglify solves the issue.

    Running "uglify:dist" (uglify) task
    >> 1 sourcemap created.
    >> 1 file created.

    Running "uglify:polyfillOnly" (uglify) task
    >> 1 sourcemap created.
    >> 1 file created.