Force number of devices to 1 so as not to break the awk script.

The awk snippet that grabs the cpu stats assumes that there's one disk
in iostat's output. Adding -n 1 to the iostat call ensures that iostat
will only report on one disk, and the cpu stats will be in the correct
column.
Merge pull request #2 from lglenn/use_iostats_on_mac

Force number of devices to 1 so as not to break the awk script.