try this:
phonegap plugin add https://github.com/phonegap-build/ChildBrowser#f4d5ef2ed6f35da0af8a819f30edcd53e6877770
UPDATE: I just tested again with the above command. It still works. I am very new to git so I might be wrong in this explanation... I found that the most updated version of this plugin is not the master branch. Then I proceeded to get the "hash" (or tag?) of the most updated branch from the url of github (which is "f4d5ef2ed6f35da0af8a819f30edcd53e6877770"). Because I wanted to download that particular version for my local build. After digging from the internet, I found that I could put append "#f4d5ef2ed6f35da0af8a819f30edcd53e6877770" (note: with that "#" in the beginning) in the url for the command. Then it worked.
If someone tries to build it with phonegap's cloud service, it's okay to just include this tag in config.xml
<gap:plugin name="com.phonegap.plugins.childbrowser" />
which you can find in the docs.