Remove Play Grunt integration

- This is hopefully a quicker, less painful fix than trying to get the plugin
  working

- To keep the assets updated locally you will need to run `grunt` alongside the
  app, once you have done the `npm install -g grunt-cli && npm install`

- The Jenkins job will also require reconfiguring as a consequence of this
Merge pull request #5 from alphagov/remove-play-grunt-integration

Remove Play Grunt integration