gpg: check for S.gpg-agent to see if gpg-agent is running

from gpg changelog:

>  Removed the GPG_AGENT_INFO related code.  GnuPG does now
>  always use a fixed socket name in its home directory.

This is basically the same fix as PaBLoX-CL posted in https://github.com/sorin-ionescu/prezto/issues/689