build: Update grunt-contrib-uglify to v0.6.0

Fixes output to be non-empty.

Before

> Running "jshint:dev" (jshint) task
> >> 89 files lint free.
>
> Running "uglify:js" (uglify) task
>

After

> Running "uglify:js" (uglify) task
> >> 4 sourcemaps created.
> >> 4 files created.

Change-Id: I1eb402d4ca1e96bd9082ccb2417f36dc452e61ce