Documentation: added ruby version

Hi,

I struggled to get it working on a centos 6 machine. It turned out the default ruby version installed from the standard repositories is 1.8.7 and therefore I got this error:

  check_puppetdb.rb:72:in `doRequest': uninitialized constant Timeout (NameError)
	from check_puppetdb.rb:84:in `commandProcessingMetrics'
	from check_puppetdb.rb:249

Once I installed ruby 1.9.3 the check_puppetdb worked fine as it should be!

I added the ruby version 1.9.3 to the README file just as a reference for other user :)