add json dep

knife from chef client versions > 11.12 dies on 'knife backup restore {clients|users}' with the error "Exception: NoMethodError: undefined method `create_id=' for JSON:Module"

chef/json_compat.rb dropped its stdlib dep after 11.12, and its native JSONCompat library doesn't have the create_id method. In chef-client > 11.12, you can drop the create_id and use load() instead of parse(), but that doesn't work in older versions.  "require 'json'" seems to be the simplest fix compatible with older and newer clients.