hledger-install should warn about or upgrade old stack

[simonmichael]

hledger-install.sh uses any stack executable that's already installed. Fairly often that is too old and fails. We need to check the stack version and do something helpful if it is older than what's recommended on download page (currently 1.7.1; usually the latest release is the most likely to succeed). Eg: warn, terminate, show upgrade instructions, offer to try upgrading with stack --upgrade if it's 1.5 or newer, try upgrading it without asking, offer to rename/delete old stack executable(s) and try again, or do that without asking... A PR for any of these would be great.