Improve the update task to download jQuery 2 as well.
Update the existing assets re-downloaded from jQuery CDN.

These assets were downloaded from the `http://code.jquery.com/jquery.js` related
URLs, but now they are from the http://code.jquery.com/jquery-VERSION.js` ones.
Add jQuery 2 assets.
Update VERSIONS file to include jQuery 2 version
Update README
Merge pull request #170 from rails/jquery-2

Ship both jQuery 1 and jQuery 2.