Add installcheck_script to check for "json" Gem

* Added an installcheck_script that runs a simple ruby script to
attempt loading the "json" gem. On error, it aborts (and returns 1
preventing Munki install).
* This should, hopefully, help end users of the Puppet AutoPkg recipes who are deploying to versions of OS X < 10.9 avoid having Puppet break on those clients due to the missing "json" Gem.