Bump to a version of ivy that will not swap in httpclient URL handler.

When ivy finds the httpclient on the classpath, it will silently change
its url handler, and the httpclient in ivy 2.3.x has a few known
issues.
Add notes for this fix.
Merge pull request #1602 from sbt/wip/ivy-no-commons-httpclient

Bump to a version of ivy that will not swap in httpclient URL handler.