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. 
Yes we do need to add it, as I said it was my understanding that Modernizr was doing it for me. I'll be happy to just add:
```javascript
<script>
// Picture element HTML5 shiv
document.createElement( "picture" );
</script>
```

When we do actually use the picture element for the talks page.