exim: force Gmail over IPv4

We have reports of outgoing mails of ours getting false positives on
their Gmail account (both wikimedia.org Google Apps and gmail.com).

Others in the admin community have mentioned that using IPv6 to talk to
Gmail increased the odds of getting a bad spam reputation, an effect
that lasts for a while.

Turn this off for now for both gmail.com & Google Apps primary MXes. We
can later disable in a more controlled fashion once we've cleaned up SPF
& added DKIM, in case these increase our odds.

(manually applied to mchenry as well)

Change-Id: I42a63a82ff4c411fd2378f1bf1b0383b46b8b728