use HZ_100 as suggested by @tatsushid

see #148 for more information:

After many trials of changing VirtualBox parameters, I unexpectedly
found "Local timer interrupts" of /proc/interrupts grew in a short time.
So I rebuilt boot2docker ISO with kernel_config

CONFIG_HZ_100=y
CONFIG_HZ=100
instead of

CONFIG_HZ_300=y
CONFIG_HZ=300
Those values are recommended at
http://www.virtualbox.org/manual/ch12.html#idp57194208 . After replacing
original ISO with my custom one, the problem seems to be solved. Before
that, I could hardly pull docker images but now it's ok and other
commands work properly.
Merge pull request #354 from SvenDowideit/pr_out_use_hz_100_as_suggested_by_tatsushid

use HZ_100 as suggested by @tatsushid