Remove the gzip reporter.

It uses a dependency which requires node >=0.12. On node 0.10 (used in the
travis builds) this reporter break the buid with a

    Object #<Object> has no method 'gzipSync'

This reporter needs the `--verbose` flag on grunt to display its
result, this commit won't change anything to the default output.

Gruntfile.js

@@ -115,7 +115,6 @@ module.exports = function(grunt) {
     },
     uglify: {
       options: {
-        report: 'gzip',
         mangle: true,
         preserveComments: false,
         banner : grunt.file.read('lib/license_header.js')