readme: correct mentions of localhost to www.packer.io
readme: correct mentions of localhost to www.packer.io

cc/ @mitchellh

smerrill/packer,msabramo/packer,suma/packer,Kaixiang/packer,suma/packer,suma/packer,patricklucas/packer,smerrill/packer,smerrill/packer,TranscendComputing/packer,suma/packer,patricklucas/packer,TranscendComputing/packer,TranscendComputing/packer,msabramo/packer,msabramo/packer,patricklucas/packer,patricklucas/packer,smerrill/packer,TranscendComputing/packer,Kaixiang/packer,zyegfryed/packer,zyegfryed/packer,Kaixiang/packer,zyegfryed/packer,zyegfryed/packer,msabramo/packer,Kaixiang/packer