exim: security bump to version 4.86.2

Fixes:
CVE-2016-1531 - All installations having Exim set-uid root and using
'perl_startup' are vulnerable to a local privilege escalation. Any user
who can start an instance of Exim (and this is normally *any* user) can
gain root privileges. If you do not use 'perl_startup' you *should* be
safe.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>