use HZ_100 as suggested by @tatsushid

from #148 , @tatsushi
> 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.