Lcg/yay chef 12

1.9.0, 1.9.1, and 1.9.2 are all EOL, too. So we should rev to at least 1.9.3, I think. Given the impending EOL of 1.9.3, I think we should either pre-emptively drop 1.9.3 also, or make it official policy that we drop ruby versions when then EOL at latest so we're not stuck supporting EOL stuff in the future. I raised this issue on the relevant chef-rfc, so I don't think we need to decide here, but we definitely should require at least 1.9.3.