Undefine property error on child if compass not installed.
It happend on using concurrent task on yeoman
On win7 if you didn't have compass installed it failed with a undefine property error .
With this shows a proper message that reminds you to check if you have it installed .
Merge pull request #111 from TudorCampean/undefined-property-error

Undefine property error on child if compass not installed.