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?
`HOMEBREW_CELLAR.subdirs` is an array of Pathname objects; the `basename` map makes an array of string objects like ['arpack', 'atk', ...], and `.include?` tests if the exact string "gfortran" is in the array. How would you like to see this with `each`?