Updated grunt-contrib-jshint to 0.8.0 (jshint 2.4.0)

The 0.4.3 release uses JSHint 1.1.0, whereas 0.8.0 uses the latest 2.4.0 release. This is especially useful since JSHint 2.x has EcmaScript 5 as the default (along with supporting some ES6 stuff).