If you `brew install` a formula that's already installed you get:
Warning: ripgrep-0.5.1 already installed
If you `brew install` an outdated formula that's installed you get:
Error: ripgrep-0.5.1 already installed. To install this version, first
`brew unlink ripgrep`
Instead, suggest that the user should `brew upgrade` in this case. If
the formula isn't outdated use the previous message.