Fix jquery.min.js via non-cdn

Due to `./` in  `src="./javascripts/libs/jquery.min.js"`, this was causes 404 requests, as it was using relatives paths on all subfolders.

At the same time, I didn't like the logic (felt double negative to me), so altered it to match [HTML5 boilerplate](https://github.com/h5bp/html5-boilerplate/).