Make Ohai JRuby compatible

Right now, Ohai depends on yajl to do the JSON parsing. This has native dependencies and hence does not work in JRuby. The second problem is that JRuby does not support fork and hence open4 cannot be used to execute commands by the plugins.

In this patch, I use "json" gem if we are in JRuby. I also use java.lang.ProcessBuilder instead of open4 to execute the plugin commands when in JRuby environment. Tweaked a few tests to make sure they do the right things in both MRI and JRuby.

Fix the gem spec so that we get a ohai-jruby gem or a ohai-gem based on which ruby we use to generate the gem.