support for cron on darwin os

[DZariusz]

When run under cron, machineID returns error:

exec: \"ioreg\": executable file not found in $PATH

This PR makes it will try again with absolute path instead of failing.
I was able to figure out thanks to: crontab-doesnt-execute-ioreg-on-my-mac

[dogancanbakir]

Getting a similar issue when running cmd without abs path! Thanks for the fix.

@denisbrodbeck,
Hey there, could you please merge this pull request? Thanks!