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If you `brew install` an outdated formula that's installed you get: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the formula isn't outdated use the previous message.