I see, thanks.
This is a Hadoop 0.23 bug, not a Pig issue. Theoretically Hadoop should be listing commons-httpclient as a dependency in its pom, it should come in from ivy via transitive dependencies, and automatically get on the classpath.
In fact, that's happening: if I remove this dependency from ivy/libraries.properties, it still shows up in build/ivy/lib/Pig when I build for hadoop 20, but not if I build for hadoop 23.
Should be fixed upstream...