Fix cmake invokation

[mave113]

For some reason the 'sh' python module decides to return a string in response to a Command() invokation instead of a RunningCommand instance that the code from this module expects. By setting the '_return_cmd' option to True we request the 'sh' module to always return a RunningCommand.

[ghost]

I should have commented that I ran into this issue using 'sh' version 2.0.2 while using 'init' to setup a new tutorial