Don't use jspm to load Picturefill

Unfortunately jspm throws a parse error when trying to load the Picturefill polyfill. @OliverJAsh believes this to be a bug which I will be submitting to jspm. The module has a UMD definition at the bottom of the page, and the jspm cdn version thinks this to be the case, but this error is thrown:

```javascript
Uncaught AMD module github:scottjehl/picturefill@2.0.0/src/picturefill did not define 
```

Therefore, we are temporarily going to load Picturefill with an old'skool `<script>` tag until the issue has been resolved. 
We're using a custom build of Modernizr which doesn't have a shiv: https://github.com/guardian/developers-site/blob/master/src/js/modernizr.js#L2 I opted out because we only really care about HTML5 browsers for this site.

Looking at their documentation, I think I'm still right in saying that we would need to shiv the PICTURE element if we started to use it, because of the `async` attribute on this SCRIPT element:

> If you add this attribute [`async`], you'll need to add a line of script before the script tag as well to allow older browsers to recognize picture elements if it encounters them in the page before picturefill has finished loading.

In any case, this isn't a problem. I would prefer to load the polyfill asynchronously and shiv manually (if needed) than to load this synchronously.