Do not generate a gzip report from uglify:dist.

We currently discard the gzipped output, and this is solely used to
display the results to the screen.  This adds between 5x and 10x to the
total time required for the `uglify` task.

To see the full `gzip` report, run `grunt uglify:report`.
Merge pull request #1719 from rjackson/dont-waste-my-time

Do not generate a gzip report from uglify:dist.