.mailmap: Consolidate entries for Andy and Greg

See MAPPING AUTHORS in git-shortlog(1) for details on .mailmap.

For both authors I've selected the email address that they've used
most recently.

.mailmap

+Andy Boughton <abought@gmail.com> <pchemist@gmail.com>
+Greg Wilson <gvwilson@software-carpentry.org> <gvwilson@third-bit.com>