No longer using net-http-persistent Ruby gem

Looks like net-http-persistent is causing failures sometimes. Testing
if things will be more stable without it.

Updated release notes for 0.2.21-0.2.24.

Releasing patch version 0.2.24.

Paired with: Chris McMahon
Bug: 66451

Change-Id: Ia8cc9454768cdcc1557f99aafc47e22ce7466550