When Syck is not present at all, stub it out and proceed

In some circumstances (at the very least, a clean install of JRuby 1.7.6 running in Ruby 2.0 mode), there is no YAML::Yecht name defined, nor is there a YAML::Syck. This causes safe_yaml to fail to run in those circumstances. Stubbing this namespace out (the same thing rubygems does) seems to resolve it.