(PUP-3352) Remove zlib setting

Previously, puppet had a zlib setting to control whether to use zlib
compression when connecting to the forge or when using b64_zlib_yaml
as a network serialization format.

This commit removes the zlib setting, and instead, if ruby supports
the zlib library, then we will accept compressed content, e.g. when
downloading modules from the forge.

The b64_zlib_yaml format is being removed in Puppet 4, so that part
doesn't matter.

The zlib setting defaulted to false, so there shouldn't be any
behavior changes other than you can no longer set the property to
false.