Update Rails version recommendation on README
Update Rails version recommendation on README

perobertson/shipit-engine,perobertson/shipit-engine,Shopify/shipit-engine,Shopify/shipit-engine,perobertson/shipit-engine,perobertson/shipit-engine,perobertson/shipit-engine,Shopify/shipit-engine,Shopify/shipit-engine,Shopify/shipit-engine