ipmitool: Bump ipmitool to 1.8.15

New release fixes a number of bugs, including some needed for talking
to modern AMI BMC firmware as shipped on some POWER8 systems.

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>