Theme Installer: Revert to proxying through PHP for WordPress.org API requests.

This is to ensure we have valid installation nonces, though we've run into this as a problem previously (see #27639, #27581, #27055).

A tad slower, but we gained speed in 3.9 by simplifying the request made to the API.

props ocean90.
fixes #27798.


git-svn-id: https://develop.svn.wordpress.org/trunk@28126 602fd350-edb4-49c9-b593-d223f7449a82