Warn on install if gfortran formula is installed

Take agatti's good advice in #133 and attempt to warn away users with the old gfortran formula installed.

Poking in HOMEBREW_CELLAR feels wrong but checking Formula['gfortran'] doesn't work because the formula was removed from the repository. Is there a more proper way to do this?
Is `map` really necessary here? Perhaps `each` instead? Also I would tend to avoid `include?` and would directly `equal?` because in Cellar there are no version numbers and just "gfortran" should be matched.