Don't tell homebrew users to upgrade using self-update. #82
Description
The "new version available" notice currently instructs the user to run grit self-update, even if grit was installed using homebrew.
NOTICE: An update is available, run /usr/local/bin/grit self-update to install version 0.7.1.
Actually running grit self-update then instructs the user to use brew instead:
❯ grit self-update
grit was installed via homebrew, use 'brew upgrade grit' to upgrade
The NOTICE could be improved by providing brew-specific instructions immediately.