Don't load json until after the app has a chance to load it

The current behavior loads the version of json bundled with ruby (In the case of ruby 1.9.3 anyway) before the app has a chance to load a potentially newer version of json.

The bug can be verified using this gist https://gist.github.com/jigfox/2007656

As you can see, it doesn't seem to matter what version of JSON is specified in the Gemfile because Nack loads json before the Gemfile gets evaluated.

This fix delays the requiring of json until after the Gemfile had a chance to be executed.