Just out of curiosity: what limitations of Ruby 1.9.3 is critical in your case?
Not to start a flame war, but according to my experience, Ruby 1.9 is already quite good, later versions of Ruby only introduce minor syntax & semantic changes, which is trivial to work around. This is nothing like the big differences between 1.8 and 1.9
I agree with package availability problem of mruby, tho.