Switch to non-deprecated suggested "retrive keys when verifying" option

The gpg --auto-key-retrieve option was deprecated GnuPG 1.0.7 in 2002,
in favor of --keyserver-option=auto-key-retrieve:

  http://git.gnupg.org/cgi-bin/gitweb.cgi?p=gnupg.git;a=commitdiff;h=f8f52d8

Switch to listing the non-deprecated form.