Simplify install by using local Grunt installation

Local Grunt was added in 438beae6410a3a1489530c16124cc0b5bcb23142 .
This will simplify the process of just running tests. If you want
to use grunt without having to type node_modules/.bin/grunt, you
can just run:

    $ npm install -g grunt-cli
Merge pull request #76 from alphagov/simplify-install-by-using-local-grunt-installation

Simplify install by using local Grunt installation