build: Work around jshint failure for global JSON object

build: Work around jshint failure for global JSON object

jshint is now failing with errors about how it doesn't
recognize the global 'JSON' object, even though we have
browser:true set. This worked in 2.6.0 but broke in 2.6.1.

Other people have reported similar but different problems with
jshint: https://github.com/jshint/jshint/issues/2213 .
It looks like 2.6.1 is a seriously broken release, but we
can't work around it in package.json because grunt-contrib-jshint
depends on 2.6.*, assuming that point releases are backwards
compatible. Which they should be.

Change-Id: I85bcf1c97f2f8c82e139ebf2f1b6c4248b72f45e