boxen-git-credential: handle /usr/local Homebrew.
boxen-git-credential: handle /usr/local Homebrew.

Don't assume the Git credential location as Homebrew might be in
/usr/local and due to Boxen being Boxen we can't get the location in
this script itself (as far as I can tell).

Closes #727.

DennisDenuto/our-boxen,DennisDenuto/our-boxen,DennisDenuto/our-boxen