taskd.pl: increase ref-change interval to 2

taskd.pl: increase ref-change interval to 2

Sending messages too fast can result in errors from the
SMTP server and/or end up getting the messages marked as
spam because they're being sent too quickly.

Alternatively, messages sent too quickly may be delayed
for some time before being released resulting in an awkward
out-of-order delivery.

Increase the delay between ref-change update emails
to a minimum of 2 seconds instead of 1.

With this increase, the maximum send output becomes
30 messages per minute instead of 60 thereby dropping
below a common 50 messages per minute maximum threshold
found on a number of SMTP servers.

In practice the delay would normally be this long anyway
for complex repositories due to the processing involved
to generate the message, but for smaller repositories
this will prevent the 50 messages per minute maximum
from ever being exceeded.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>