Add option to use GPGTools with pass

The homebrew-provided version of GPG is quite difficult to configure,
and Homebrew Cask provides a nice option to install GPGTools (which
takes care of a lot of configuration). This changes the pass formula to
skip installing the Homebrew gnupg2 package if gpg is already installed
through some other method (most likely GPGTools).
pass: update 1.6.3 bottle.