(#22474) Update forge/repository.rb to conditionally include zlib

Previously using the --no-zlib flag or using puppet on a ruby without zlib
support would result in a load error and stacktrace, because
lib/puppet/forge/repository.rb requires zlib and uses a constant defined by
Zlib. This commit updates repository.rb to only require zlib and use its
constant if the zlib feature is available and the --no-zlib flag was not passed
(zlib is enabled by default in defaults.rb).
With this commit running puppet on a ruby without zlib does not throw a
stacktrace or other error related to zlib.
Merge pull request #1886 from haus/ticket/stable/22474_conditionally_require_zlib

(#22474) Update forge/repository.rb to conditionally include zlib