Installer script: don't use git if it's too old

This is an issue which came up in Tigerbrew: https://github.com/mistydemeo/tigerbrew/issues/9

Old versions of git can't clone from Github via HTTPS, but right now we assume that if we find any version of git at all that it's safe to use it. (We still need a fallback for if we find a compatible git version but it fails for some reason. There's no real error handling.)

Mainly putting this out for feedback because I'm terrified of killing the installer script, heh.