Upgrade picturefill from 2.2.1 to 2.3.0

Follow-up to #2260061: Responsive image module does not support sizes/picture polyfill 2.2

From the author's mouth:

I think 2.2.1 was short-lived as 2.3.0 was scheduled for an imminent release. I've created two pull requests to update the site to reflect that 2.3.0 is the recommended stable version.

2.3.0 fixes a range of bugs and has some performance improvements, as well as adding support for newer image formats, intrinsic image dimensions, and commonJS exports. More details here: https://github.com/scottjehl/picturefill/releases

Basically, we heartily recommend you upgrade to 2.3.0. It's the bee's knees.

https://github.com/scottjehl/picturefill/issues/474#issuecomment-88410346

Also, 2.2.1 was never tagged in Git, yet that's the version listed in core.libraries.yml