Fixes some issues with picture that have to do with the latest spec

Status: **Opened for merge**

Reviewers: @noahadams @jvaill 

## Changes
- Fixes issues with examples using the picture element in the latest Chrome that supports the element natively.

## How to test-drive this PR
- Try the examples http://localhost:3000/mobifyjs/examples/resizeImages-picture-element/index.html and http://localhost:3000/mobifyjs/examples/resizeImages-no-capturing/index.html, both in a browser that supports the picture element (latest Chrome), and in one that doesn't (Firefox).